There is a knocking & sloshing sound from the rear. Why?

After driving through severe rainstorms, deep puddles, etc. water can collect in the base of the passenger B pillar. Ordinary amounts drain naturally but large amounts take too long. Remove the back seat and interior armrest panels and below the seatbelt reel there is a small hole in the body that accesses the flooded chamber. I used about 2' of 1/4" sprinkler hose taped to a section of wire to reach the water and attached to a wet/dry vacuum to remove it.