What is this noise in my water pipes?

Question

I can hear what sounds like pressure building and then it stops. It's almost like the sound of water being turned on but there is no water running. Sometimes it's louder then others and the length of time varies but it is never longer then a minute.

I have city water through my township and there is a gauge in the basement hooked to the mainline. When the noise occurs I can see the pressure build to almost 100 psi and then it just releases. This happens multiple times throughout the day.

I have tried bleeding the pipes thinking that there is air in them but that didn't seem to help. Also the water flow is normal and never fluctuates. I am at a loss for what to do.

I can call a plumber but I am almost certain that it won't happen while they are hear and I am not paying top dollar for someone to stand around and wait. The last plumber I asked said it was a leaky toilet and gave me a hack to determine which one but neither seem to have a problem.

Answer 1

It might be your water heater relief valve discharging when the line pressure gets too high. It's there to relieve pressure in the tank if the water gets too hot, but will also act on high pressure. Generally the outlet of that valve is plumbed to go directly into a drain, so you never see any water flowing.

Answer 2

If the pressure gauge is on the curbside of the PRV you should call your water company. Even 100 PSI is low on the curbside and could indicate a supply problem especially when it's fluctuating. Normal from the curb is 120 to 150 PSI. But it can be lower because pressure does drop depending on distance from the water supplier. If the pressure gauge is on the domestic side of the PRV I would guess that replacing the PRV could solve your problem. They do go bad and it can result in pressure fluctuations as you describe.

Edit
Domestic water pressure should be in the 60 to 75 PSI range.