Pipe noise - not water hammer
Copper pipe "thump" noise occurs intermittently (approx. every 20 seconds) and diminishes after approx. 2 hours following shutoff of hot water tap in kitchen. The noise is not location specific and is loud for the first hour and a bit. The noise resembles the sound you hear when pipes are expanding or contracting but is much louder at first. After approx. 2 hours, the noise can only be heard every 40 seconds or so then stops. My house was built in 1957 and only recently started making this sound. I'm convinced it is not water hammer yet have not been able to determine what it is through resources on the internet. Suggestions?