I have something similar happening. My home is only 18 months old. The upstairs bathroom has a double sink, but when the water is run in the sink closets to the door, it sounds like water is hitting against the outerside of the downstairs shower wall. (Downstairs is a one piece garden tub.) I also hear this same sound along with residual dripping for several minutes when the upstairs toilet is flushed. Since I can only look for moisture spots around the garage wall, that's what I've been watching for. I haven't seen any yet but I'm not sure if that means anything. My plumbing is still under warranty for six more months. Would it be wise to contact the plumber?

Hi Darlene and Welcome to The Plumbing Page at AMHD.com. You're responding to a 6 year old dead threads so I gave you one of your own. Please check the date before you post. Thanks
This may just be your pipes expanding and compacting within the walls.
Knocking, popping, creaking and crackling pipes are a common complaint, especially in colder weather. I can tell you what's happening and take the mystery out of it but you're not going to like the repair. When a draw is made on a hot water line the line expands against a pipe strap next to a stud and when you drain hot water from your tub or shower it goes into a chase that is a little cooler then room temperature. The heat expands the pipe causing it to rub against the stud to which it is pipe strapped. This is the sound you hear. As it cools it contracts and the noise is heard again. To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight. Most people when they learn what causes it just elect to live with it. Good luck and thank you for rating my answer. Tom