Bradford White water heater, I was going to flush the tank but on opening the valve a short burst of 3 cups of very dirty water with a lot sediments.

The sediment that can build up in the bottom of a water heater can actually harden the popping sound your hearing is from the water that is trapped under the build up. when your heater fires the water under that build up has no where to go so it turns to steam so the popping sound is that steam bursting through the build up. If flushing it didn't help the only way to stop it would be to get a new heater

the tank needs flushed out ! heres how. Water heater manufacturers recommend flushing sediment from your storage type water heater periodically.
If your water heater is gas, set the gas valve to "Pilot" to prevent the burners from coming on while you are flushing it. If your heater is electric be sure to turn off the circuit breakers. With an electric water heater, if the water level drops below the heating elements and the thermostat turns the elements on, the heating elements will probably burn out quite rapidly. Connect a garden hose to the drain valve at the bottom of the tank, Close the shut off valve on the cold inlet to the water heater. open the temperature/pressure relief valve at the top of the tank by lifting the lever. Leave the valve open. Open the drain valve at the bottom of the heater allowing the water to flow out through the garden hose. when the water is clear your finished. close pressure relief valve on top of tank and turn water back on.
Open a hot water faucet in your house, and let it run until no air bubbles come out.

Turn the heater back on, and with gas units re-light the pilot light if necessary. good-day !

Water heaters are a huge field of knowledge. Problem is not 100% clear.
There is a resettable thermal switch on the combustion door. This switch is round and has two wires that come from gas control valve. Thermal switch trips when combustion parts have not been cleaned and they cause overheated condition, or sensor reads temperatures above 180, or thermal switch has gone bad, or gas control has gone bad.
http://waterheatertimer.org/images/Whirlpool-Resettable-ECO-8.jpg
Add a comment with more information for continued help.

1) Bradford White support:
http://www.bradfordwhite.com/supportserv.asp

2) Bradford white mi403s6fbn is 6-year-warranty 40 gallon tank-type gas water heater with Icon electronic gas valve, and atmospheric venting.
Older models have the dial-type Robert Shaw or White Rodgers gas valve.
Open following link to see age of Bradford White:
http://www.bradfordwhite.com/findaserial.asp

3) Bradford white has 'product manuals' and 'service manuals' on following link:
Product manuals have general information and advisories that people can read about water heaters.
Service manuals are for plumbers and technicians, and do-it-yourselfers for testing and repairing water heaters.
http://www.bradfordwhite.com/documentation/residential/gas.asp

4) Typical manual for your product shows different gas valves, lighting instructions, and advisories, and troubleshoot checklist:
http://www.bradfordwhite.com/images/shared/pdfs/manuals/238-44219-00J.pdf

5) Add a comment with following information if you want specific help with servicing water heater.
What are you trying to reset on the water heater?
What are the symptoms, and what have you noticed, and what have you tried so far?
Which gas valve does your water heater have? Please open typical manual and view images of the three basic gas valves.
When was water heater last serviced and cleaned?
How old is water heater?

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Valve is not identified.
Cold water shut-off valve, TP valve, check valve, mixing valve, drain valve are the usual suspects associated with electric water heater.
Gas water heater has additional valves: gas shut off valve and gas control valve thermostat.
Check valve, TP valve and mixing valve probably need replacement.
If drain valve is leaking, put garden hose cap over end.
Shut off valve probably has a bit of blockage, or is not completely open. Try closing and then opening shut-off valve all the way, and if problem persists, replace or clean out valve.

What kind of noise? Are you sure the noise is from a valve or is it inside the tank caused by sediment build-up inside of tank?

http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/How-to-replace-TP-valve.html
http://waterheatertimer.org/images/Water-heater-with-mixing-valve-300.jpg

Replace overflow valve to end the drip.

If tank type water heater has some hot water, then upper element is receiving 240V and electric circuit can be eliminated as suspect.
Lower element or shorted wire to lower part of tank or upper thermostat not wired correctly are next suspects. If plumber replaced the parts the wiring will be correct, but he may not have checked for shorted wire.
If the elements were replaced but sediment was not cleaned out of tank, then that can also cause the problem you describe: the new lower element is shorted out because its stuck in the sediment or no water is circulating around the new element.
Open following links for details how to test and clean tank and test elements:
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html

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You posted under electric water heater.

1) Gas or electric, the noise is from sediment build-up inside tank.
Once sediment builds up, the only way to solve problem is to clean the tank out.

2) To restore full efficiency to electric water heater and stop the noise, clean the tank:
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html
3) To clean gas water heater is more difficult and less successful:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

4) To prevent sediment problem, flush water heater each 6 months before sediment can build up.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html#flushelectric

5) Other things to know for water heater maintenance.
http://waterheatertimer.org/Replace-anode-rod.html
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

Information below is for tank-type electric or gas heater only.

There is a condition called stacking.
This might apply to your water heater or not.
Having authorized service people to your house 3 times suggests more complicated mix of problems.

Stacking is caused when there are repeated short draws of hot water.
As you know, each time any hot water is drawn out of faucet, cold water immediately enters bottom of tank through the dip tube. The dip tube is connected to cold water inlet and runs to bottom of tank.
So all cold water enters bottom of tank close to the gas water heater thermostat.
See illustration of dip tube, that also shows copper thermostat sticking into tank that is connected to gas valve.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html#flushelectric

Let's say your water heater if fully heated and burner is off.
A short draw of hot water causes cold water to enter tank.
Another short draw. And then another short draw, each draw happens close to same time as other draws.
Water at top of tank is plenty hot, but bursts of cold water get near thermostat and cause thermostat to turn on burner.
Burner heats water and more hot water rises to top of tank. Water at top of tank becomes overheated each time stacking occurs.

Again, a stacking situation may not be the cause of your problem.
However, remember if single-handled faucet is opened straight upward, then that draws both hot and cold water. And cold water immediately enters tank.
Only when the single-handled faucet is lifted with the handle all the way to the right, does it draw only cold water. Even so, some single-handled faucets have bleed-over between hot and cold sides, so hot water is drawn anyway.

Another possibility is that the dip tube ends at or near thermostat, instead of going all the way to bottom of tank. The dip tube should go straight down from cold water inlet, but dip tubes are made of plastic, and some self-cleaning tanks have a curved dip tube or dip tube with perforations that create currents designed to stir sediments off bottom of tank (although this is not proven to be effective deterrent to sediment formation).

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I do water heater research for specific purpose.
I see complaints about every brand except the brands that are no longer in business
Most complaints about gas heaters involve burners & pilot.
Here are some things that might help you decide:

New water heaters have to meet regulations for clean-burning and efficiency.
As a result, water heaters exposed to poor incoming air supply, and poor venting are more likely to be affected by chronic pilot light problems.

Bradford White manual I was reading said, to remove chemicals from same room as water heater. Chemicals corrode water heater parts. This would affect the pilot light as well.
'Spray propellants, cleaning solvents, refrigerants, swimming pool chemicals, calcium and sodium chloride, household bleach, waxes and process chemicals are corrosive at very low levels with no odor."

Yearly maintenance: you're supposed to clean burner and combustion area with a wire bush and vacuum each year.

The FVIR is another consideration today. Flammable Vapor prevention.
Consult the manual before buying the product.
Many gas water heaters have a screen located on bottom of water heater. When the screen clogs with dust, the reduced airflow will stop the burner. Unless the water heater legs are set on bricks, then you can't get under to clean the screen.
If the FVIR trips because it detected a flash-over condition, you have to buy parts for all water heaters except the Bradford White. BW has a re-settable FVIR.
http://www.thetankatwaterheaterrescue.com/forums/forum3/2679.html
http://411plumb.com/bradford-white-defender-water-heater-review

The 411plumb,com website is a good source of information
The Tank is a good place to post water heater questions: Those guys are professional plumbers.
http://www.thetankatwaterheaterrescue.com/forums/forum3/

Maintaining your water heater is important.
Look for a brass drain valve instead of plastic.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Anode rod.
If water heater has two anode rods, the tank will last longer.
Inspect anode rod every 3 years.
Loosen anode rod before installing tank so it will come out easily for inspection
Or if it has a powered anode rod, it will supposedly last 'forever' ... until the burner gives out.
http://waterheatertimer.org/Replace-anode-rod.html

Other water heater ideas:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

Oct 2010. If you have more questions, add a comment at bottom and it sends me an e-mail
There are a lot of good sources in the links above.

yes you can get a universal one at any hardware store[about 15$]. shut off cold valve at top of tank and drain tank through drain at bottom of tank.you only need to drain enough to get the water level below the t&p valve. you should use dope and teflon on the new threads of the valve to ensure no leaks. a big cresent wrench or pipe wrench will work to unscrew the valve in a counterclockwise direction

I believe you have a high water pressure going to the
tank. Minimize the water volume going to your tank. This is done by fully closing the inlet valve under the tank. Then open about 1/4 to 1/3 of the inlet valve. To do this, count the number of turns to get it full close from full open position. For 1/4, divide the number of turns by 4 and divide it by 3, for 1/3 open. Hope this will solve your problem.