Hello,
I have a 2 story home with copper throughout. In our upstairs bathroom, we hear a dripping sound when the hot water to the sink is on. I have tried to figure this out on my own and have learned the following:
1. This only occurs when the hot water faucet is on.
2. This does not happen when the shower hot water is on.
3. It is not from the drainage line since it only occurs when the hot water is on.
4. I opened the back side of the bathroom wall (luckily the bathroom backs up to the garage) and I checked for leaks in the pipe, but found nothing.

The only place I can pinpoint the sound coming from is the top of the black pvc drainage pipe where it goes through the ceiling. I tried to inspect from the attic, but it is where the roof slants and I cannot get close enough to see.

Is this just the pipe expanding and contracting, or a sign of something else. Thanks and sorry for the long explanation.

It is most likely the copper pipe expanding from the hot water. The pipe actually gets a bit longer when it heats up and they can rub againist some framing making a sounds like a loud sharp drip.

I agree with ballengerb1, it is called expansion and contraction. Not much you can do about it unles you can get to the pipe or pipes that are making the noise.