When i use either the sink or bathtub in our main bathroom, as it's draining there is a knocking in the drain pipe that continues as long as 5 minutes after we are done running water. It happens worse when you use straight hot water and it's worse on the sink. When i put my hand on the pipe in the basement where it comes down through the floor i can feel it but the pipe really doesn't move much. I was thinking maybe a venting issue but everything drains great and never bubbles, gurgles, or surges as it's draining. There is another bathroom on the same drain line but it does not cause the issue. Started happening about 2 months ago. I'm stumped!