Hot Water Drain Making Noise

[Packerman]

Our upstairs master bathroom sink makes a noise that sounds like dripping behind the wall. I opened up the wall in a few spots and there is no leaks. The drain pipes are made of black plastic. The noise only occurs with hot water. I know it is the drain because I have filled the sinks with hot water and you do not hear it while the sinks are filling. The noise occurs when I drain the hot water. I felt the pipe and you can feel a small amount of vibration.

Have any of you had similar problems or have any idea what is happening? Are the pipes expanding with the hot water?

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[DA]

no leaks. The drain pipes are made of black plastic.

feel a small amount of vibration.

I have a very similar issue, also in an upstairs bathroom, although I would describe the sound as rather loud clicks or bangs of one solid material against another, not dripping. I think in my case the issue is the 2-story-high straight drop of the drain pipe just before a 90 degree turn - the water drops 10-12' down in irregular splashes and then hits the bottom really hard making the pipe move a little and bang against a bracket (which I'm yet to identify) or a stud somewhere inside the wall.

I'd be honest, it never occurred to me to open the wall even though the phenomenon is really annoying (and can probably cause a leak at some point), so I'm just wildly guessing at this point. However, nothing of the sort happens in the lower level bathroom or another one upstairs that has drain pipes laid out differently.

If someone has a better idea about what could be happening to make those noises in drain pipes, I would love to hear that as well.

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