I recently had plumber repair broken joint where vanity drain goes into main vent stack. He used rubber "mission" couplings above and below the new vent stack joint as well as where the vanity drain goes into the vent stack. Everything drains fine with no leaks. He had to access this from the closet of our bathroom, but cutting a 2' x 3' hole or so. I left the hole open for a week or so just to make sure everything was working fine. After this time I went ahead and covered the hole back up with drywall. Now, when someone runs water in the vanity, you hear this tapping noise coming from that area. The tapping noise even happens when the shower is used. The funny thing about this is that the shower drain goes into the vent stack at least 2' lower than where the repair was made. The tapping seems to be more pronounced when hot water is drained. The drains are all working fast and there is definitely no leak. The joint he repaired is the highest one in the line. All I can think is that maybe he should have used mission couplings with full metal sleeves and maybe the rubber expands/contracts from the heat? But like I said, the tapping noise happens when the shower drains, with the shower joint at least 2' below the repaired joint. Any ideas? Thanks.

Most likely, the drain below the vent connection was moved slightly during the repair, and is now contacting a wood structural member. When hot water passes thru the drain there is expansion and the resulting creaking noises that accompany it.

You may be hearing the expansion of the water lines that you couldn't hear through the drywall before. as the pipe expands and moves, it rubs against the strapping holding it in place with a kind of stick-slide stick-slide that leaves you with the ticking as it moves each time. Once the wall is repaired you probably won't hear a thing anyway, but I would throw some insulation in the wall cavity to dampen the sound.

It for sure is expansion of the ABS or PVC pipe from hot water. I would open the wall and make sure there is clearance around the piping and stuff felt between the pipe and the wood so it cannot rub against the wood. Make sure felt is taped to the pipe if you think it will rub against drywall or wood. Note the felt has to completey surround the pipe where it passes through the plates or fire blocks. If you have a 90 degree fitting on top to break away from a property line, make sure that it has felt on top so it can't rub against anything.