Cracking noises from PVC pipes

Every time I flush a toilet or drain a bathtub or sink from either of the two bathrooms on my second floor, I hear "cracking" noises from the PVC drain pipes. It is very loud at the beginning of the water draining and then tapers off near the end of the draining. This noise has been going on for months. It happens all the time. There are no water leaks It's just that the PVC pipes make "cracking" noises for lack of a better description of the noise.

My two questions are

1. What causes the noise?
2. How can I eliminate the noise?

Thanx in advance for any advice offered.

That is normally pipe being pinched by the wood. Pipe moves and the wood doesn't. You can go in and make sure the wood is not snug against the plastic pipe.

PVC actually changes size quite a bit with temperature changes, so this is especially prevelant when running hot water down the pipes - it heats up quickly, expands, and anywhere it is tight or constricted, rubs against things and makes noises. It makes noises when it contracts after the hot water stops, too, but often that is slower, so you may not hear it, or hear it as much.

Thanx for the info. I was getting a little concerned that it might be some sort of serious structural issue affecting the PVC pipes which would have to be addressed althought the house is only eleven years old.

Thanx again.