I am very confused about a noise coming from my exhaust. When I start the car, usually after it has been sitting for a while, I hear a loud rumble from the back, and then once I start moving, there is a rumble and a squeak together (squeak sounds like metal on metal), but it stops after 20 feet of driving. I just moved and had the pipe that is connected to the exhaust manifold and before the cat replaced before I did so. The rest of the system in back was good, it's fairly new. What could this be? I had the exhaust system inspected by what I thought was a good mechanic out here, but he couldn't find anything. It's really starting to freak me out.