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?

Water hammer occurs when you shut off a faucet so I agree it's not water hammer. Sounds more like expansion and contraction. Knocking, popping, creaking and crackling pipes are a common complaint, especially in colder weather. I can tell you what's happening and take the mystery out of it but you're not going to like the repair. When a draw is made on a hot water line the line expands against a pipe strap next to a stud and when you drain hot water from your tub or shower it goes into a chase that is a little cooler then room temperature. The heat expands the pipe causing it to rub against the stud to which it is pipe strapped. This is the sound you hear. As it cools it contracts and the noise is heard again. To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight. Most people when they learn what causes it just elect to live with it. Regards, Tom