Check under the seat. There is a rear floor heat duct that exits through a rectangular oppening in the floorpan under the seat. There is enough room around this duct for small items to enter. You would be amazed at some of the crap I've found in this area. You will need a good flashlight and a small mirror to look in to this area. It is a sheet metal sttructural member, and when stuff gets in it don't come back. Sometimes helpfull to have someone drive the car, steer side to side, accelerate, brake, to make the item roll around while you look through the oppening from the back seat. Things I have found, golf balls, pens, lipstick, marbles, petrified chicken mcnuggets, just to name a few. And yes, it can sound like its in the roof, back of car, under car, etc. Good luck.