Knocking within drain pipe

[nnayak]

After using the bathrooms on the 2nd floor (i.e. flush toilets, use the sink, etc.), there is a knocking sound in the drain pipe inside the wall (near the top of the 1st floor). The knocking speed is faster when the flow is heavier (say when flushing the toilet).

The knocking occurs even when the water supply is shut off and the toilet flushed. I think this means it is not water hammer in the supply pipes.

There is no problem with draining the water though and it flows down the drain free. The vent pipe also appears to be free of any obstruction.

Could it be that the drain pipe is banging against the inside of the walls?

Any ideas on what might be the problem and how to fix it.

Nitin

[Mike Swearingen]

Yep. Sounds like a loose drain pipe knocking against the wood framing.
Only way to stop it is to open up the wall and secure it right.
Good luck!
Mike