Any reason why this pressure release valve would be squealing like this?

It’s a house with a random pressure relief valve sticking out of it connected to a CPVC line sticking out of the house. It’s not connected to the water heater or anything. Just generally attached to the water system. Can I just cap the damn thing?? It squeals like a pig whenever someone uses water. What’s the risk in capping this thing? Excess pressure will fuck every fixture in the house or what? And no, the water heater has no expansion tank. Should I cap this and put in an expansion tank?