Hopefully someone here knows what I'm talking about and I didn't just make the whole thing up as part of a fever dream.

I recall reading a book several years ago where one of the main plot points was that there was some kind of AI watching and recording humanity across the galaxy, but had been doing it for so much longer than designed that it was starting to break down or run out of storage space or something of that nature. It influenced some guy to head out and start looking for it to help or fix it. I don't remember much else, and all of the search engines just spam me with garbage sponsored AI bait, so I'm hoping someone here might recognize the broad strokes and be able to point me in the right direction.

I have a very odd issue that I can't solve, perhaps someone here could help. The pipe attached to my main shutoff valve is emitting a soft knocking sound at around 120 beats per minute. The sound persists as long as there is any amount of pressure, and I can feel it in the pipe nearby as well as in the valve handle itself. The weird part is that if I turn on any hot water tap, the noise vanishes. I'm a new homeowner so I don't have much experience, so any help would be appreciated. What I've checked so far:

Drained the system completely and refilled

Turned the water off overnight and checked toilets for leaks

I don't feel the pulse at the water meter line

Turning hot water on anywhere in the house stops the noise.

Thanks in advance!