Loud Noise Behind Walls After Water Shut Off

Hi Everyone,

I've been looking for information to help me understand my plumbing problems. What a wonderful forum this is!

My wife and I moved in to this 10-year-old single story house (sitting on slab, with water heater and A/C in the attic) a couple weeks ago. At our first night there, we heard these huge banging noises coming from behind the walls between the master bath and the hall bathroom. It lasted an hour or two that night. First I thought it was caused by the water heater. Went up to the attic while hot water was running in the kitchen. Didn't hear any noise.

I started noticing the noises occurred after we took showers. I thought it was caused by the pipes not having been used daily during the 9 months this house was on the market. So I poured a bottle of Drano down the master shower. The noise seemed to have gone down some and lasted shorter. I was hoping another bottle of Drano would get rid of it. It might be a wishful thinking.

After reading about all these water hammer, air trap, and moving pipe issues online, I'm not sure if I've a more serious problem. I would like to get some idea what the problem is before hiring a plumber. Any recommendation on how to diagnose this problem will be deeply appreciated!!!

CW

Unless I totally missed that day of class, DWV systems are not pressurized therefore not subject to water hammer.

Generally, water has to be flowing for a noise to happen unless there is something living in your pipes. The only case I can think of is a freeze-thaw cycle going on in a relatively flat run in the dwv if the water is not flowing. If your nick means what I think it means, there ain't no freezing going on where you are

Not a pro so this is a layman guessing.

SP

If you have a pressure relief valve (and if you do, you should have an expansion tank), if the expansion tank has died, after using hot water, the water expanding has nowhere to go. If the PRV is getting old, it can make noises as the water expands. Normally, it would hold, and then the expanded water would dump out of the water heater's T&P vale. I vote for the PRV needing service/replacement. But, I'd look for an expansion tank first. If you have neither of those, it could be a checkvalve at the water meter.