Rear end makes a knocking noise when decelerating at about 40MPH. when serviced they said it wasn't the brakes nor calipers but one bearing is slightly loose.

I would have to say that your front wheel bearings are no good and need replacing that is the usual sound they make and the pitch changes threw out the driving speeds and corners I would say.

TROUBLESHOOTING ENGINE NOISES
A KNOCKING SOUND IN THE ENGINE IS MOST LIKELY CAUSED BY ONE OF THE FOLLOWING
1. PISTON SLAP: Makes a sharp metallic noise. Idle engine and short out each cylinder plug. The noise will disappear when plug with bad piston is shorted. Noise will also disappear at acceleration. This can be caused by worn or out of round cylinder, or broken piston ring. Correct problem by re-boring cylinder and/or replacing piston.
2. VALVE NOISE: Makes clicking or rattle noise. Caused by excessive wear on valve stem or lifter, out of adjustment, or stuck valve. Correct by adjusting valve clearance, replace worn valve or lifter, regrind cam, replace valve guide and /or valve. A stuck valve can sometimes be loosened by passing oil through the carburetor while engine is running.

3. ROD BEARING KNOCK: Makes sharp metallic noise similar to a piston slap. Detection is opposite of piston slap. Rod knock is not heard at idle. Knock becomes louder as engine speed is increased. Caused by excessive rod bearing clearance. Correct by adjusting rod bearing clearance to .0015 inches by removing shims. May require re-pouring rod bearing.
4. REAR MAIN BEARING KNOCK: Makes dull knocking or thud noise. Detected at speeds between 20 and 50 MPH. Knock will normally decrease or disappear while pulling or decelerating. Noise will be detected the loudest at normal driving speed, when not pulling or decelerating. Correct by adjusting bearing clearance to .001 to .0015 inches. If knock is excessive, crank should be checked for out of roundness. May need to re-pour all main bearings to correct.
5. TIMING GEAR KNOCK: usually the most difficult to diagnose. If gear is loose or badly worn it will knock in all ranges. Run engine slightly above idle speed. Slowly open and close throttle. Knock will continue to be present, but just as engine slows down knock will become a slight rattle. Remove timing pin and reinsert into timing hole on timing gear cover. Press timing pin tightly against timing gear and accelerate slightly above idle. Knock will significantly be reduced or disappear. Correct by replacing both timing gear and crank gear as a matched set. The two gears should have a backlash clearance of .003 to .004. If more than .009 inch backlash, an oversize (.005) timing gear should be installed.
6. WRIST PIN SLAP: This can not be detected by shorting out the cylinder plugs. Rapidly accelerate and decelerate the engine speed. The engine will pass through a certain speed range when the wrist pin will rattle at about the same pitch as a valve tappet noise. This can be corrected by installing a new wrist pin bushing in the rod or new wrist pin f badly worn. Wrist pin should fit the piston and connecting rod with a tight metal to metal fit. The pin can be pushed into the piston and rod with a slight pressure of the hand. Pin to rod clearance is .0003 to .0005 inches.

A KNOCKING SOUND IN THE ENGINE IS MOST LIKELY CAUSED BY ONE OF THE FOLLOWING
1. PISTON SLAP: Makes a sharp metallic noise. Idle engine and short out each cylinder plug. The noise will disappear when plug with bad piston is shorted. Noise will also disappear at acceleration. This can be caused by worn or out of round cylinder, or broken piston ring. Correct problem by re-boring cylinder and/or replacing piston.
2. VALVE NOISE: Makes clicking or rattle noise. Caused by excessive wear on valve stem or lifter, out of adjustment, or stuck valve. Correct by adjusting valve clearance, replace worn valve or lifter, regrind cam, replace valve guide and /or valve. A stuck valve can sometimes be loosened by passing oil through the carburetor while engine is running.

3. ROD BEARING KNOCK: Makes sharp metallic noise similar to a piston slap. Detection is opposite of piston slap. Rod knock is not heard at idle. Knock becomes louder as engine speed is increased. Caused by excessive rod bearing clearance. Correct by adjusting rod bearing clearance to .0015 inches by removing shims. May require re-pouring rod bearing.
4. REAR MAIN BEARING KNOCK: Makes dull knocking or thud noise. Detected at speeds between 20 and 50 MPH. Knock will normally decrease or disappear while pulling or decelerating. Noise will be detected the loudest at normal driving speed, when not pulling or decelerating. Correct by adjusting bearing clearance to .001 to .0015 inches. If knock is excessive, crank should be checked for out of roundness. May need to re-pour all main bearings to correct.
5. TIMING GEAR KNOCK: usually the most difficult to diagnose. If gear is loose or badly worn it will knock in all ranges. Run engine slightly above idle speed. Slowly open and close throttle. Knock will continue to be present, but just as engine slows down knock will become a slight rattle. Remove timing pin and reinsert into timing hole on timing gear cover. Press timing pin tightly against timing gear and accelerate slightly above idle. Knock will significantly be reduced or disappear. Correct by replacing both timing gear and crank gear as a matched set. The two gears should have a backlash clearance of .003 to .004. If more than .009 inch backlash, an oversize (.005) timing gear should be installed.
6. WRIST PIN SLAP: This can not be detected by shorting out the cylinder plugs. Rapidly accelerate and decelerate the engine speed. The engine will pass through a certain speed range when the wrist pin will rattle at about the same pitch as a valve tappet noise. This can be corrected by installing a new wrist pin bushing in the rod or new wrist pin f badly worn. Wrist pin should fit the piston and connecting rod with a tight metal to metal fit. The pin can be pushed into the piston and rod with a slight pressure of the hand. Pin to rod clearance is .0003 to .0005 inches.

A KNOCKING SOUND IN THE ENGINE IS MOST LIKELY CAUSED BY ONE OF THE FOLLOWING
1. PISTON SLAP: Makes a sharp metallic noise. Idle engine and short out each cylinder plug. The noise will disappear when plug with bad piston is shorted. Noise will also disappear at acceleration. This can be caused by worn or out of round cylinder, or broken piston ring. Correct problem by re-boring cylinder and/or replacing piston.
2. VALVE NOISE: Makes clicking or rattle noise. Caused by excessive wear on valve stem or lifter, out of adjustment, or stuck valve. Correct by adjusting valve clearance, replace worn valve or lifter, regrind cam, replace valve guide and /or valve. A stuck valve can sometimes be loosened by passing oil through the carburetor while engine is running.

3. ROD BEARING KNOCK: Makes sharp metallic noise similar to a piston slap. Detection is opposite of piston slap. Rod knock is not heard at idle. Knock becomes louder as engine speed is increased. Caused by excessive rod bearing clearance. Correct by adjusting rod bearing clearance to .0015 inches by removing shims. May require re-pouring rod bearing.
4. REAR MAIN BEARING KNOCK: Makes dull knocking or thud noise. Detected at speeds between 20 and 50 MPH. Knock will normally decrease or disappear while pulling or decelerating. Noise will be detected the loudest at normal driving speed, when not pulling or decelerating. Correct by adjusting bearing clearance to .001 to .0015 inches. If knock is excessive, crank should be checked for out of roundness. May need to re-pour all main bearings to correct.
5. TIMING GEAR KNOCK: usually the most difficult to diagnose. If gear is loose or badly worn it will knock in all ranges. Run engine slightly above idle speed. Slowly open and close throttle. Knock will continue to be present, but just as engine slows down knock will become a slight rattle. Remove timing pin and reinsert into timing hole on timing gear cover. Press timing pin tightly against timing gear and accelerate slightly above idle. Knock will significantly be reduced or disappear. Correct by replacing both timing gear and crank gear as a matched set. The two gears should have a backlash clearance of .003 to .004. If more than .009 inch backlash, an oversize (.005) timing gear should be installed.
6. WRIST PIN SLAP: This can not be detected by shorting out the cylinder plugs. Rapidly accelerate and decelerate the engine speed. The engine will pass through a certain speed range when the wrist pin will rattle at about the same pitch as a valve tappet noise. This can be corrected by installing a new wrist pin bushing in the rod or new wrist pin f badly worn. Wrist pin should fit the piston and connecting rod with a tight metal to metal fit. The pin can be pushed into the piston and rod with a slight pressure of the hand. Pin to rod clearance is .0003 to .0005 inches.

Several possibilities. Sound transmits through metal. The rear Emergency brakes will lose their lining and the linings fall into the rear brake drum. They will often pulverize the material and the piles of debris will shift when backing up and repeat the process going forward.
Rust on the front rotors. Constant battle. Can rust in 3 days enough to make noise. Sticking caliper or rubber brake hose is breaking down inside and blocking brake fluid return. Fluid has pressure to tighten caliper but relies on gravity/suction return. Rubber brake hoses internally can form flaps which act like 1 way gates.
Check other front axle for same parts, maybe 1/2 shaft is failing.

have brakes checked,and wheel bearings,sounds like pads bad or caliper problem.

Hello, some of the Front wheel drive cars used a rear suspension system which requires an Alignment just the same as the front end. You can get the rear Alignment separately but it is usually a packaged 4 wheel Alignment. Jack up the back end of the car and turn the wheels and check for them turning hard. This will indicate a brake or bearing problem.

The rear wheels do have an Emergency brake which can cause problems dragging even when not used. The bonded lining falls off the brake shoe when it rusts and the lining falls onto the rotating parts and gets ground up. Sometimes you will hear a noise like loose rust as you stop and start at low speeds. You can also have a sticking rear Caliper or drum brake.

The rear hubs have bearings which can fail; usually you will hear noises or have vibrations when the bearings are worn loose. Your best bet is to remove the rear brake rotor and visually inspect the Emergency brakes and feel the turning of the rear hub bearing. If you do not have the rear brake assembly on the car it will be easier to work on the bearing. Bearings are made 2 ways. The Sealed bearings are replaced as a hub unit. The Serviceable bearings have removeable bearings and races.

You want to have the rear bearing in good shape before having an Alignment. The Sealed bearing hubs are the easiest to replace by removing and replacing only bolts. The Serviceable bearings can require punching out and installing a new race, then packing the bearing with grease.

Some shops do both Brakes and Alignment and some just do the brakes. You can fix what you can handle before getting an Alignment or pay for both jobs. I hope you find my solution very helpful.

Was this a front or rear wheel? Did it make any unusual noise prior to the problem? If front, you could have a brake caliper that is sticking which would cause the brake to stay "on" and overheat the brakes or you could have a wheel bearing that has failed and is locked up but they usually make some noise before they fail. If rear, then usually it is brake related...again a caliper or wheel cylinder could be the culprit. It's not often that a rear bearing fails but that is possible.
If you have a brake caliper that has gone bad then I would suggest replacment of both calipers, brake pads, and flushing out the old brake fluid so you'll have good calipers and clean fluid when you are done.

Id get a mechanic to look at it. Sounds like a rear end bearing problem. Axle bearings pumpkin bearings.

hiya this does sound iffy ? but b4 i comment /offer an opinion could you or your friend jack up the wheel in question that has had repair and look at how the brake disc is secured to the actual hub assy that wheel bearing is housed in ? also caliper? cracked?? is it now a new one? have never had problem renewing front or rear bearings in past using the correct tool/s a hydraulic press is the best method for removing excessive siezed components as for your friend now saying noise is from rear ? it may be pads worne or disc/s corrosion/scoured so investigate and update what you discover ? sounds like big hammer garage?