What is this noise in my water pipes?

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.