From your description, sounds like the beginnings of calcification of water heater to me. I'm not a pro......but I've heard what I think are the sounds you're describing.

Ok...here is an update. There was no thermal expansion tank on this system, so one was installed. It did not fix the noise problem. In the course of installing the tank, it was discovered that the supplied water to the house is over 100 psi. There is a pressure reduction valve, but adjusting it seems to make no difference in pressure, so I'm guessing that valve could be bad and need replacement. Based on this additional info, does anyone have any other ideas about causes of and/or fixes for this noise problem?

Okay...

First things first: You must fix the pressure issue in order to eliminate that from the list of problems.

Second: how old is the water heater and do you have hard water (or do you have a water softener)?

Jake is right on the money...

Service pressure needs to come down and PRVs do fail so if it can't be adjusted it needs to be replaced. IIRC code is 80psi and usually in house pressure is about 60psi.

An expansion tank is code required if the plumbing is a closed system and with a PRV it probably is a closed system.

If you haven't been routinely draining your WH at least once a year then the noise could be sediment/calcium (hardness) build up in the WH over 10 years.

Thanks, guys. I just ordered a re-build kit for the PRV. After that gets fixed, I'll do another update.

Ok...here's the final update. We took the pressure reducing valve apart (that's another story...one involving an internal steel nut corroded on a threaded brass stem that held the diaphragm in place, and required a nut splitter to remove). After installing the new o-rings, diaphragm, etc., and reassembling it, the water pressure got reduced from 110 PSI to 80 PSI...and the noisy, rumbling pipe noise disappeared. I'm guessing that the PRV was trying to do its job, but couldn't anymore and was shuddering as it moved against the worn down rubber seals.

IIRC code max pressure is 80psi so you might want to lower yours a bit to 60psi.

I haven't had much luck rebuilding PVRs. You might want to keep an eye on your pressure for a while. A good investment is a pressure gauge with a tattletale needle so you can see pressure spikes. They can be had that will screw on to a hose bib. They used to sell these at HD... don't know if they still do.

justalurker-
Thanks for the comment about your experience with PRV rebuilds.

I would have replaced the valve, but it is soldered into a location that is terrible for access. I'm not sure whether trying to rebuild it, though, was more work than cutting it out and re-routing the plumbing run. The plumber -- not me -- mounted it so the backside was buried in the sheetrock and it's soldered in place so you can't remove it easily. The plugs in back could only be accessed by cutting out part of the wall and maneuvering wrenches around nearby pipes. If the re-build doesn't hold up, replacement will be my next project.

I do have one of those valves and will keep an eye on it. Thanks.

Now you have two things to watch (worry) for... leaks from the repaired PRV and high pressure returning.

For peace of mind I'd have to replace the PRV and while I was at it make it easier to R&R in the future.