Its a pretty bad rattle. Sometimes it sounds like a loose marble in there, but its not always consistent. It will rattle really bad then sometimes it will quiet down. It changes up from park, drive, and reverse. Reverse makes it rattle the worst. When the engine is still warming up and I've just begun to drive it it makes a different type of horrible noise if I accelerate too hard, but if I let off the gas it stops. After its up to temp the horrible noise goes away. The rear main seal was going, but it was still holding oil alright.