After shower and/or hot water fauset at sink is turned off, I hear this slow dripping noise in the wall behind the sink. This is upstairs. I checked ceiling below this area and it is not wet. Before I bought this place, it had frozen pipe damage but was repaired some 5/6 years ago and haven't had any problem until now.

Are you finding signs of water pooling ANYPLACE? It might not be there, but even on another floor some distance away... or eleswhere on that floor.

Could be expansion and contraction of the piping, particularly on the hot water side.

I am with ma on this but can't give him more greenies for now. Expansion of pipes inside a wall sound very much like a drip

I'm thinking it's the expansion thing after I read several other posts of this problem - Whooo, I'm feeling better! Thanks for your responses!