Re: Knocking Drain Pipe
Five minutes is quite a long duration where 30 seconds upward to a minute is more common.
I agree with johnjh20 regarding the expansion and contraction of the drain line rubbing being the common problem. Typically it is more of a clicking ( ticking ) sound that's heard.
However, a knocking sound points to a loose pipe that is not supported properly. When large volumes of water drain, the pipe will move and bang against the studs within the wall or even the wall itself if there is any space or against joists if run under the floor.
It may be at the point where the sink drain pipe goes through the wall or the floor and may connect to the 2 inch line from the tub. Or the 2 inch tub drain pipe itself is loose or bellied. Check the drain line from the tail piece on the sink to see if it's moving down when you drain the tub.
Another thing you may find --- supply pipes and drain pipes that run right next to each other are striking one another and creating a clatter.
2 cents worth.
"" an ounce of perception -- a pound of obscure "