Plumbing – that dripping noise I hear in the wall whenever cold water is used after sitting awhile

I live in a condo…upstairs with another unit behind me with the plumbing in the shared wall between us. Whenever it's been awhile (say 45 minutes to an hour) since any cold water has been used, if I flush either of 2 toilets or run any cold water faucet, I hear a dripping sound that is loudest in the wall of the kitchen.

It starts about 15 seconds after the water starts (flush or open faucet), drips (tapping sound) very quickly and then slows down and stops. After that, I won't hear it again until the water has been sitting unused for awhile. It's only cold water. The only thing I hear when hot water is used, is the slight knocking from heat expansion after it's running for 30 sec. (it's winter).

There is no apparent water leak upstairs in the kitchen or downstairs in another unit. This just started about 6 weeks ago. It's almost as if water is accumulating somewhere in the pipe and using the water pushes this accumulation out of the way.

I don't know if it has any bearing (or just coincidence that it started around the same time), but I believe the vent to the roof has something blocking it, as my toilet water levels (especially 1 toilet) go down at varying rates. Sometimes I'll come home and find a tiny little something that backed up into the toilets. A plumber has already checked for a toilet leak and it's all dry. I'm waiting for the Spring until someone goes up onto the roof to check that out.