Bad vibration coming from under floor

Over the last six months I started hearing a cracking/popping sound coming from under the van on every bump. I comes from bump I hit no matter the wheel. It has an sensation of the floor warping or wobbling as I can feel it through the floor under my feet. At first I thought is was the spare tire under the van's rear shaking but it is not loose. The exhaust pipe does not seem to be the problem either. It has gotten much worse now and I have not a clue what it is.