Are you sure its from the back? I've heard of a plugged A/C condensate drain making that sound. Are the from carpets ever damp?
How long since you've taken a good long drive (over an hour)? Water is a result of combustion and can condensate in mufflers. (Ever see water come out exhaust pipes?)
How long since you've taken a good long drive (over an hour)? Water is a result of combustion and can condensate in mufflers. (Ever see water come out exhaust pipes?)