That just doesn't sound right (so to speak).
Are you sure there isn't a leak somewhere.
1) Get an assistant
2) When the car is cold, start it, and have the assistant place a (gloved) hand over the tailpipe to seal it as best they can, while you crawl under the car to feel for leaks at each joint. If there are really no leaks, it will be very difficult for the assistant to keep a hand on there at all, and the car may even stall.
Keep in mind that 'torquing' the exhaust about in the back during the install can sometimes cause things to break loose in the front, suchas at the headpipes.
One last thing- some newer mufflers have a drain hole. I have no idea on MBs. These are supposed to extend the life of the muffler by allowing water to drain, but I have found they allow enough exhaust out to make some extra noise. I thought my new Audi muffler was defective! If the new one has this, and the old one did not, that could explain what you are hearing.
53k on a 1991? Do you ever leave the house? Selling anytime soon?
1986 300E 5-Speed 240k mi.