I just had a new toilet installed. When I flush toilet and the tank fills and fill valve shuts the water off, I hear a dripping sound in the wall for about 15 to 20 seconds. Then it stops until I flush the toilet again and it repeats.
I had a drip sound like this. Showed up as a small puddle on the basement floor. Turned out the sewer drain had a T installed near landing on the basement steps/side door; an old sink?. Ripped into the plaster and lath wall, and found a piece of 90 year old lead pipe sticking out of the T. The lead had been crushed shut, but decayed over the decades. Cut out about 10 Lbs of lead, and screwed a cover over the remaining end of the T. No more drip. Replacing the lath and plaster hole was hard. Probably should have hired that out. Except it's pretty much a lost art. What I have looks OK, but there's no mistaking it for drywall, or the work of a skilled plasterer.
The lead, cast, is now serving as a couple of nice 5 pound ankle weights.