Shrieking noise from under bonnet

There are 2 possible reasons for this, one is the water pump bearing is defective, the other is the timing belt tensioner pulley bearing is defective and the other is a defective timing belt idler pulley, has the main accessory drive belt been removed to see if one of those pulley bearings is at fault?

Hi,

If the noise is under the bonnet and it occurs with the car stationary, use an old length of fuel hose as a stethoscope. Place one end up near to your ear and move the other end towards where you think the noise is coming from - it'll be much easier to pick its source this way. But remember to watch out for whirling fans and drivebelts when you're waving the hose around!
First Checks If something that is supposed to be lubricated makes a noise, check that it actually is lubricated! So with a noisy gearbox check the oil level, with noisy suspension check that ball-joints are greased, and with a noisy power steering pump make sure that the fluid is up to the right level. At the same time make a visual check - are there any leaks, obvious broken parts, loose nuts or bolts?
Engine Noises here are often pretty serious. But that makes identifying the cause of them even more important, before there's a chance for major, major damage to occur.
If there's a high-pitched 'ting, ting' sound at high loads or in a high gear at low speeds, the engine is detonating (knocking). It's a sound that is a bit like coins rattling together, and it indicates that there's unstable combustion occurring. Either the fuel octane is too low, the ignition timing is too advanced, the mixture is lean, or (in a forced aspiration car) the boost pressure is too high.
Change them! Don't rely on the (pictured) knock sensor to save your engine - assuming it even has one, that is!
A strong, dull noise that increases when the engine is revved may be caused by the crank's main bearings knocking. Cause a misfire one by one in each cylinder (take off a plug or fuel injector lead) and see if the noise stops when the cylinder is unloaded. If it does, the un-powered cylinder is the one causing the noise. The big end bearings knocking make a similar noise, but a little higher in pitch. Again disable the cylinder to see which one is causing it. The final bearing which can cause problems in the reciprocating assembly is the small end around the piston pin. This noise is heard when the piston is at the highest and lowest ends of its travel. Find out which cylinder it's coming from using the cylinder disable technique.
Each of the above noises sounds from deep inside the engine, and so is a muffled 'banging' type of sound, not really mechanical but more like a cloth-wrapped hammer blow. Piston slap, however, is heard as an overlapping series of metallic-sounding noises. It is usually louder when the engine is cold, but gets quieter with increasing engine temp. Often when this noise is present there will also be heavy oil consumption.
Each of the noises is very serious - the car should not be driven and - if it has to be - not revved hard. A full engine pull-down and overhaul is normally indicated by any of these noises: unfortunately, it's bad news!
A high-pitched loud squealing that increases in pitch as engine revs climb is probably a slipping drive belt. This will happen more often when the engine is cold, and simply requires tightening of the belt. Noise from the water pump area normally means that the pump is stuffed and needs replacement.
If you car has manual tappet adjustment then there may be a rapid 'tap-tap-tap' noise, easily heard at idle and increasing with engine revs. This means that a tappet adjustment is required - easy stuff.
If your car has hydraulic tappets and it still makes this noise, one or more of the hydraulic lifters has died and needs replacing. Note that in an EFI car the injectors can make a very similar noise - use the rubber tube stethoscope to listen to one and then confirm that they all make the same noise.
Clutch Clutch noises happen when the clutch is being used - unlike gearbox noises which can happen in neutral as well! A clutch which is noisy whenever the pedal is pushed down may have a worn-out or unlubricated release bearing. Note that this isn't just a squeak from the pedal (just lubricate the pedal if it is!) but instead is a noise that is continuous with the pedal down.
Noises that come as the clutch is released are all caused by the clutch assembly having worn or broken bits in it. If the noise is accompanied by a grabbing or chattering clutch then it'll have to come out. Note that a squealing clutch can sometimes occur if you've had a 'paddle' high-performance unit fitted. This doesn't mean that anything's wrong - it's just the noise that the friction material makes.
Gearbox In an auto box signs of distress are normally shown by poor gear changing behaviour - flares on shifts, slippage and so on. But in a manual trans, noises are often very important first signs of problems. If the noise is present in neutral, push in the clutch. If the noise doesn't stop, check out the engine and clutch.
If the noise stops, it's the tranny - but the first step is still to adjust the engine idle speed to make sure thaShrieking noise from under bonnet - 107750_5lo.jpgh.
A roaring noise that occurs with the gear lever in all positions and which gets loudest at a certain engine rpm may mean that the gear teeth are worn. A rhythmic knocking sound from the trans indicates that there are broken gears or bearings - a trans pull-down will be necessary. Noises that occur during gear changing indicate that the synchros are having trouble. The first step is to check the clutch operation, and then after that a gearbox pull-down will again be necessary.
As you can see, noises from the gearbox are always bad news! However, a box that very gradually becomes noisier over time will often continue for a good many kays without problems. It's a sudden change that warrants close attention.
Tailshaft Noise from the tailshaft will usually occur during throttle-off coasting - with a tailshaft problem, vibration is the more common symptom when accelerating! A knocking sound when driving off or coasting can be traced to worn universals (most often) or worn mainshaft splines. Both times they'll need replacement to cure the problem. Others to look out for include a damaged centre bearing (on two-piece shafts) or loose Shrieking noise from under bonnet - 107750_6lo.jpgf the tailshaft fastenings.
Suspension Noises from the suspension are usually of the 'clunk, clunk' type. If your car suddenly develops a suspension noise after you've been working on it, immediately pull over to the side of the road!! The first step is to make sure that you remembered to tighten the wheel nuts - loose nuts will make the wheel clunk as you drive along. [In 15 years of playing with cars I've forgotten to tighten the wheel-nuts twice. Both times there was enough noise to let me know of it.] In fact loose nuts on any of the suspension components can cause noises, and so check that they are all tight and that split pins (where fitted) remain in place.
Wheels can literally fall right off if a split-pin is missing and a nut becomes loose....
Make sure that all ball-joints are greased and that rubber suspension bushes (especially in older cars) haven't worn away. Pushing the corner of the car up and down will let you hear noises caused by broken shocks, springs and so on.
Grinding noises that vary with speed and cornering loads often come from wheel-bearings. One quick check of the bearings is to grab the wheel and try to shake it - it should have near-zero free-play.
HoweveShrieking noise from under bonnet - 107750_7lo.jpgar (150,000+) is likely to have wheel bearings well on the way out.
Brakes The cause of brake squeals and shrieks can often be fairly minor, but it can also indicate that drums are distorted or the friction surfaces scored or contaminated. If the noise occurs all of the time that the car is moving but disappears when you put your foot very gently on the brake pedal, the brakes may be dragging. 'Donk-donk' noises during braking can indicate that the discs are warped - but you'll usually feel that through a juddering brake pedal as well.

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