We've had both our water heaters replaced within the last 5 years. Sometime in the last couple of years we've started hearing very heavy rumblings from both of them. It's possible the noises started about the time of the last install.
Initially assumed it was sediment. Flushed the first one multiple times; including using vinegar per manufacturer's suggestion. No effect. Had a plumber come and flush it. No effect.
Sometime during this campaign, noticed the other heater was also making similar noises. Flushed it. No effect. One note that may be a hint as to the problem. Had to remove the cold water line to get this heater to drain.
Some significant information about these noises: 1) they have nothing to do with the heating of the water - they occur when water is being USED, i.e., they start as soon as water is turned on. They are more prevalent when the water being used is hot water, but also sometimes occur when only cold water is used (e.g., flushing of toilets). 2) They NEVER occur if water is not being used (i.e., we do not hear any noises when water is being heated). 3) The sound is like something (large air bubbles, turbulent water?) bumping the sides of the tank. Or, the sides of the tanks flexing?
As you might deduce, I'm very suspicious of some sort of pressure issue. But I haven't found a "plumber" who seems very interested in troubleshooting this problem.
Any ideas about troubleshooting or possible solutions would be very much appreciated.

 

City or well water?
Only two things can cause to high a pressure, incoming water pressure it to high, or over heating.
Simple enough to check the pressure any Lowes or HD sells a simple gauge you can screw onto the outside faucet.
There's no reason you should need more the 50 PSI.
Run the hot water over a thermometer. Does not need to be over 120 Deg.

 

sounds like a PRV valve issue or the water meter itself.. These would need to be checked.. as well as pressures...