Pressure relief valve failed.

We werE woken up a few nights ago by a pretty loud water hammer. Happened 2 or three more times that night. No water running so know of "spontaneous". Checked everything out and ended up fiddling with the water pressure on the PRV (an old Honeywell). This reduced the severity but still get a hammering. Interestingly it starts when you open a tap, and continues for 2 - 3 minutes after shut off. Not every time but often. My best trouble shooting guess is a bad PRV that is allowing pressure fluctuations in the line. You can feel the vibration in the feed line just downstream from the valve, and if you shut off the water supply tap the vibration stops. This is GPS g to be. 700+ dollar replacement so I'd appreciate Ny advice on how to conform my diagnosis

If is a honeywell bruckman you can get a repair cartridge remove top and old center cartridge, reinstall top and you are finished

That would great. Don't know about Brickman, it is a Do6f -probably as old as the house 1978