Water Heaters - Sounds like water running, but no evidence of leak

This weekend our pipes began making a noise as though water was running. All our inside and outside faucets were off. The noise was audible throughout the house, from one end to the other. There is no sign of a pipe break anywhere in our house, and outside also seems to be quite dry. (We have a slab foundation, fyi)

The noise stops when we shut off our water heater. The plumber who came by on Monday said he can't tell where the "leak" is without using his special diagnostic tool, and to repair plumbing in the slab could cost a few thousand dollars. Any expense right now is too much for us.

Someone mentioned to me that it could be our temperature/pressure relief valve on the water heater, but I have no idea how to test this or how this would cause our pipes to sound like water is running when it's not.

Any advice?

Your temp/pressure relief valve is located near or on top of your water heater. It is "L" shaped and brass. It may or may not have a pipe coming out of it and discharging to a remote location. When you find it, put your hand on the pipe leading from it and you will be able to tell if it is leaking, it will be quite warm.(due to hot water exiting it).If it doe's not have a pipe on it, you will SEE the water leaking from it.
An easy way to see if water is leaking is to look at the water meter. If the "hand" or a little triangle shaped thing is moving, then water is going somewhere.
I don't understand why it would stop with the water heater turned off unless it is hot water leaking (only way that could happen).
Check these things out. If none are the cause then I am stumped. Except for an in slab leak. Good luck.

The same day the noise began, I was cooking a lot and using the kitchen faucet quite a bit. At one point it seemed to me that the hot water was much hotter than usual, but I just attributed it to me using more water than usual and thus getting more freshly heated water (and thereby, hotter :o ).

Would variations in water temp indicate problems with the TPV?

It sounds like there is a leak in the hot water supply somewhere, bad news is it will be more then likely under the slab, might want to get 3 estimates to see what it will take to fix or reroute all the hot water lines to all the fixtures.

If your water was hotter than usual, then that may indicate a failing thermostat, which would cause the heater to overheat, which would cause the T&P valve to be releasing the excess hot water. OR, it might just be a failing T&P valve.
Check that first. You may just need to replace the T&P or tstats. That would be a lot easier and far cheaper than a slab leak job.
Good luck!
Mike

Update on the noisy but mysteriously invisible hot water "leak":

When we turned our water heater back on last night, the noise did not return. We ran hot water at all our sinks and in the tub, and flushed the toilet. Still, the sound of leaking water did not return. It was gone all night, and it was quiet when we left the house this morning.

I think we're definitely going to be having a look at the thermostats and the temp/pressure valve.

It could have been that or a hungup toilet running.
Mike

Are the thermostats something we can test/replace ourselves?