Muffler is getting old, but is still pretty quiet except at startup when cold. I suspect is has crud inside that has flaked off due to internal rusting, and which rattles.

Has anyone opened up a stock 535i muffler to see how it's insides are designed? I'm curious about removing the loose debris, perhaps by vacuuming, but don't want to make things worse in the process. This to extract some more miles before installing an expensive replacement.