Huge knocking sound after flush, but pretty sure it's not water hammer

You can hear the flush in the beginning, and a few seconds later a loud knock under the sink area. If I understand, water hammer is from the incoming water stopping suddenly and moving the pipes, but this is definitely elsewhere along the outflow.

Worth mentioning this only happens with solid waste and liquid doesn't do it. Kinda sounds like the outflow pipes are knocking against something when there's bulk in there.

Does this sound kind something to be remedied? Thanks for your help!