[MRU]

Have begun hearing quite noticeable "cracking" sounds in the walls behind upstairs bathroom whenever water drains from sink or shower (but not toilet). I thought it might be a problem with the water lines and wall studs but have ruled this out by filling sink/tub with stoppers in place. Noise doesn't occur until the water drains. (Don't know why it doesn't occur when toilet flushes though - different egress perhaps).

So I'm thinking it's something to do (a clog?) with the vent stack. The cracking sound also produces a detectible sensation both underfoot and by placing my hand against the drywall. It's hard to isolate the sound since it can be heard/felt anywhere from the attic to the walls of the ground floor bathroom beneath.

Any ideas of what this is, what's causing it, and what to do about it would be appreciated.

[speedball1]

Have begun hearing quite noticable "cracking" sounds in the walls behind upstairs bathroom whenever water drains from sink or shower (but not toilet). I thought it might be a problem with the water lines and wall studs but have ruled this out by filling sink/tub with stoppers in place. Noise doesn't occur until the water drains. (Don't know why it doesn't occur when toilet flushes though - different egress perhaps).

So I'm thinking it's something to do (a clog?) with the vent stack. The cracking sound also produces a detectible sensation both underfoot and by placing my hand against the drywall. It's hard to isolate the sound since it can be heard/felt anywhere from the attic to the walls of the ground floor bathroom beneath.

Any ideas of what this is, what's causing it, and what to do about it would be appreciated.

I don't know about stopping the sound but I can take the mystery out of it.
I'll bet the noise stops shortly after the drainage's complete.
When we pipe a house for water and drainage we strap the pipes to studs, rafters, floor joists, etc. These pipes are in your walls where it's a little cooler then your house temperature. When you shower, bathe or use the lavatory hot water is used. Draining the water tends to heat up the pipes as the hotter water passes through. The sound you hear are the pipes expanding and contracting against the strap and whatever they're strapped to. You don't hear it when you flush because only cold water's used so the pipe doesn't expand. I guess if you really want to stop it you could open up the wall where the sound is loudest and shim the pipes tight but it's my guess that when warmer weather gets here and the ambient temperature in the walls approaches the house temperature the noise will stop. Cheers, Tom