I don't think it could be an exhaust leak, I had a new resonator and rear muffler installed just last year. The hissing sound isn't far enough forward of the rear wheel to be the resonator and it isn't far enough on the driver's side of the car to be the muffler. In fact, if I go around to the driver's side by the rear wheel you don't hear the hissing at all.
1995 E420 - retired
1986 420SEL - retired