Odd Noise In The Wall...

[DeeKay]

I have recently been noticing a faint knocking noise in the wall of a bathroom after shutting off the hot water tap. The noise only occurs after using either of the two sinks in that bathroom and not the one on the other side of the wall in a separate bathroom. It only occurs when using the hot water tap in either sink - and only after the water is running hot which takes 20-30 sec after opening the tap normally. The noise is in the middle of the wall between the sinks and initially is a light knocking sound spaced a couple seconds apart which then slows down to perhaps 5-6 seconds and eventually a minute or more between sounds until it stops.

I had a plumber suggest that it was a mild water hammer. He found that my Pressure Relief Valve on the house supply line was poorly regulating and suggested changing the PRV and backflow preventer, both of which were 40+ years old. Those replacements along with replumbing some plastic in that utility room with copper were pretty expensive - and they had no effect on the knocking noise in the bathroom wall. His next suggestion was to install an in-line hammer arrestor in the hot water lines in the bathroom which I have not yet done.

The plumbing lines in the bathroom were installed during a remodel in 1992. I have no records from the plumber and cannot recall if that plumbing used copper or plastic, but I suspect it was copper at that date.

Can anyone suggest a possible cause for the knocking noise other than discussed above? Many thanks for any input.

 

[Terry]

That sounds like a plastic drain that expands as it's heated by the hot water passing through and then as it contracts, it makes a knocking sound. This happens when the hole is drilled at a bit of an angle and binds the pipe. The fix for me has been to open the wall and cut away the wood that is binding on the pipe. Once you fix a few of those, you become more careful on how straight your holes are, and sometimes instead of drilling with a 2-9/16" bit I pull out my 2-7/8" bit so that I have less chance of binding the pipe. It's a looser fit.

Or you can just ignore it and that works too. It's noisy, but won't matter in the long run.

 

[Sponsor]

 

[Jeff H Young]

Just as Terry said. sSunds like a classic case. Learned this one my first year in the trade very common problem

 

[Themp]

My hot copper supply line to the master bathroom will knock when you turn on the hot water and then stop while the hot water is running. But when you turn off the hot water(can be a sink, shower, or tub) it will knock again as it cools down. This has been going on for over 35 years. I have always wanted to open up the walls and find it, but tile is in the way. And who knows where it actually is happening as it is very hard to pin point.

 

[Jeff H Young]

My hot copper supply line to the master bathroom will knock when you turn on the hot water and then stop while the hot water is running. But when you turn off the hot water(can be a sink, shower, or tub) it will knock again as it cools down. This has been going on for over 35 years. I have always wanted to open up the walls and find it, but tile is in the way. And who knows where it actually is happening as it is very hard to pin point.

if you have no interest or need to re tile Id leave it alone

 

[DeeKay]

Terry (and Jeff),

You NAILED it ! I filled a stoppered sink with hot water from one of the taps. After shutting off the water there was no noise. When I opened the stopper and the sink drained, the tapping noise began. Problem Identified ! (but opening the wall to quiet the pipe is not justified...)

Thanks for your help.