Knocking sound in pipes for hours after last use

[Rob H]

Hi there,
I've recently moved house into an old property (built 1938). Something that keeps me awake at night is an intermittent knocking coming from somewhere in the walls, that I believe must be the plumbing (doesn't sound like the house 'settling').

It's not Really loud (not deafening), but very noticeable. It sounds like a cross between a knock, a tap and a drip. When the toilet is flushed or the tap is used, it comes quite rapid fire (two a second maybe). But then it continues once every 30 seconds or so (I think it's randomly timed rather than always the same gap) for an hour or two after any plumbing was last used.

We have a system boiler I think it's called - not a combi boiler. There's a cold water tank in the loft that I think supplies everything except the kitchen, and a hot water storage tank fed by the boiler. The boiler is 7.5 years old, and the radiators/radiator plumbing looks relatively new (not original 1938!). Much of the piping I can see in the loft looks rather older, though I can't be sure that piping is still used.

I've heard people suggest it could be high water pressure - I closed the stopcock a bit and it still continues.

I've heard people suggest water hammer for similar problems - it doesn't quite sound like they're describing, but it could be - would draining the system by closing the stopcock and letting the taps run, and then refilling, be a potential solution? Are there any pitfalls to watch out for in doing that?

I've also read about the ballcock in the loft cold water tank potentially being the issue, but I don't know what to do to investigate that further.

Any advice much appreciated!

 

[jtsplumbing]

Have you checked for Mice ? could have the little devils running around the cavity's, or failing that previous owner has been walled up & trying to get out.

 

[Rob H]

Thanks for the reply ;-) it definitely doesn't sound like the scampering of feet - unless they're knocking into something i suppose, but the fact that it's the same noise occurring when one's used the taps/plumbing suggests it's more to do with the plumbing than any mice. I'll keep an eye out though!

 

[sammathias]

Probably your loft ballvalve making the noise, it seems that your saying your bathroom cold taps are still feed by the loft tank?? If possible isolate the loft tank. Draw water off for a couple of minutes and see if the noise continues. Remember to turn the tank back on tho.