Hello. I have been struggling with a big plumbing issue. My home is 6 years old, and about 3.5 years ago I noticed a knocking noise in the walls while running hot water. I called out a plumber with no resolution. I have narrowed the problem down to hot water Drainage. The knocking occurs when HOT water drains from upstairs (bath, laundry room). To confirm that the problem was water drainage and not just running hot water, I plugged the sinks and tubs and let the hot water run and I heard no knocks. But when I unstopped the sinks & tubs and allowed the hot water to drain, the knocks returned. I do not hear the knock when I drain hot water from downstairs. What can be causing this and how can it be fixed.

Hey Jack,

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 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