Noise coming from water lines?

I have a strange situation with a whining noise that I believe is coming from my water lines. The noise is very hard to trace, but I believe it occurs AFTER running hot water (shower, faucett, etc.) The noise can be stopped by running either hot or cold water - it may stop for only a short time and then return. The noise only occurs for about 20 - 30 minutes, not continuously, and only after running hot water. Any thoughts on possible causes?

Put your ear up to the water heater and see if it's coming from there.

Stop at HD or Lowes or a local hardware/plumbing store, and pick up a water pressure gauge. Check the pressure when this happens...my guess is that it is peaking.

After using hot water, the cold that went into the tank expands when it is heated. My guess is that you may have a either a pressure reduction valve (prv) or a check valve, and the increased pressure is causing the sound as the water pushes itself past the valve.

It could also be the toilet fill valve...they are notorious for weird sounds when the seals are failing.

I didn't have time to stop and get a pressure gauge last night, but I did try a couple of things. The noise is definitely related to the cold water expanding as it is heated. There is a shut off valve on the cold water side of the hot water heater - when I shut it off last night the noise stopped. When I opened it, the noise really got with it for a while. Same thing when I shut the valve where the water line comes into the basement.

There is a prv located just after the main shut off valve, but I couldn't tell if that is where the noise originates.

What can I do about this - is a thermal expansion tank needed?

Tee in a small expansion tank somewhere on the house side of the PRV.