A leak in condo's bathroom pipes?

[Alex A.]

I hear an occasional drip sound in our bathroom walls - very occasional, sometimes prominent after using water.

No evidence of leak. Could it be pipe expansion/contraction or a leak?

[home improvement guy]

The pipe in the wall is most likely a vent pipe and does not contain water.

The vent pipe extends above the roof and provides atmospheric pressure to the drains under the floors so the water from the sinks, tubs and toilets can flow to the sewer. There may be a horizontal vent pipe from the sink or tub that attaches to the main vent which is usually just behind the toilet. This is a simple explanation of a venting system. There are situations where a different configuration should be installed.

To answer your question, the dripping noise could be a leak in a horizontal portion of the venting system or condensation that forms inside the vertical pipe that extends above the roof. When condensation builds up it can drip inside the horizontal pipe. In a condo, the vent pipes and drains may be shared inside the common wall with the neighboring apartment where there may be evidence of a leak.

Talk to your condo association to see if other residents are experiencing similar problems. I would not rule out the possibility of a pipe or pipe fitting expanding when the bathroom is used. If the drip noise occurs when using hot water rather when using just cold water, that could be a clue pointing toward pipe expansion or cracking.