I recently had a new high efficiency hot water heater installed, in which the plumbers needed to install pvc pipe through the room, leading outside. Ever since(2 weeks), I hear a drip sound when coming from the upstairs. I look where the sound is coming from and everything is dry, no appearance of leaks, etc. It is very annoying and bothersome to say the least. It did not happen before this was installed. Please help. Any ideas? Thank you, Jim

Heat expansion of the PVC pipe from the power vent. Most noticeable in cold weather. Pipe is contacting building surfaces and "pulls" along them. I'm sure you hear it only when power vent is running and then cooling.