Winfred ,
I'm sorry but u can call it my lack of knowledge "catback" ???, did u replace just the rear muffler the one which has the exhaust tips on the end or the whole exhaust line from the engine manifold to the exhaust tips.
I had a very loud rattle from my exhaust and a loud bang whenever I released the gas pedal at a medium to high rpm range and had set myself to do this as soon as I first test-drove my vehicle, I had it done 3 days ago the whole thing from central silencer to the rear muffler (the pic for the same is on a realoem parts page click here for the same) I managed to get 1 out from an E32 '89 750i from a scrap dealer I know who imports half-cut and complete (really clean) cars from Japan. The part looks almost new and after replacement I feel a definite improvement in power and a much sweeter sound when I push it hard and a near dead-silent idle.
Thanks,
Ali