If you have PVC drain pipes, that sound is probably caused by thermal expansion of the pipes due to warm/hot water flowing through them. Many plumbers don't leave any expansion clearance when they route the pipes through sill plates, joists, etc. When the pipes expand, they start banging against the wood. If you're lucky enough to have access to the pipes from below, you can probably see where the problem is occurring, and possibly increase the clearance. Be careful that you don't damage the drain pipe, or compromise the house structure.