When I am stopped in drive there is some times a rear rumble noise which is farily deep sounding. It does this only when in drive at a stop. Is tehre somthing lose back there rattling or? Thanks

I have that same problem too. I checked the cat by knocking with a rubber mallet to see if pellets were melted together, I also knocked the rest of the exhaust system to see if a rock or something got in there and nothing. so I am stumped. need help too. Have you tried these? most common is catyalic pellets melted together in one ball cause knocking nooise.