Water Heater Noise Question

I"m got a hot water heater at work that is making this god awful sound that is resonating(sp) up through the floors. I'm not at work right now and I don't have all the information on it but I'll try to give the best descriptioin possible.

The water heater has got to be at least 100 to 150 gal maybe more. It serves our main kitchen area and the showers in the fitness center. It is electric, has an expantion tank and a circulating pump installed. The sound, for lack of a better distription, sounds like a chain being pulled through the whole system. It is not a constant sound but it's like the guy pulling the chain takes a break for a few minutes and then gives a big yank. We can't tell exactly where the sound is coming from. We have changed out the circulating pump being that it's the only thing with moveable parts and we still are getting this noise.

We've been working on this for about a month now on and off. Any suggestions would greatly be appreciated.

Does it go away if you shut off the recirculation loop?

Does it happen only during heating with the lower element?

noise

The pump is not the only thing with moving parts. The check valve has a flap that can bounce up and down under the right conditions. One of them being if a pressure drop can overwhelm the pump's flow causing the flap to bang shut.

Water heater noise

For all plumbers out there if you have a Commc. 100 gal. Or more water heater it pretty much means it has more then one lower element,
on this kind of water heater i have seen anywhere from two up to 9 or more
elements,


The noise problem is eather caused by check valve as noted above
or could be caused by lime buildup in tank and on elements which is what i think "REDWOOD" was getting to