u/HeavyRadio5109

Back story, new water heater installed by orevious owner. I found expansion tank bad, replaced with same unit. Adjusted tank for water pressure. (Matched the slightly more than 80 psi) weeks later, while showing I can hear the pipe thumbing like crazy. Rhythmic. It's the pipe exiting the water heater. Obvious if you shut off the water. It stops. Only does it for 15-20 seconds roughly. This only just started happening a few days ago and does always happen.

Any insight would be appreciated!