Our upstairs master bathroom makes a noise in the wall that sounds like dripping. The noise only occurs with hot water and it is located in the drain pipe not the supply line. I know this because I plugged the sink and filled it with hot water. i then turned off the water and when I drained the hot water it started making the dripping sound again. I opened up a small area in the wall and there are no signs of leaks.

My best guess is that it is thermal expansion. If this is the case why am I not having this issue anywhere else in the house? Would rubber expansion joints help the situation? The pipe is solid black plastic not PEX. It is rigid and you can sort of feel a small vibration when it makes the drip noise. My theory is that the expansion joint would decouple this eliminating the vibration. Does this theory make sense? Has anyone else experienced this type of problem? If so, how did you resolve it?

 

Somewhere in the wall or floor a hole where the pipe goes through is too small. When the pipe expands from hot water you get a clicking/dripping sound. You need to find that hole and make it bigger or put a shim in it.

Mike NJ