[Hib Halverson]

Your 91 has the early style ZF and they have a lot of gear rattle in neutral, even with the dual mass wheel. In fact, one reason the later cars had a redesigned trans with gears having increased helix angles was to decrease gear rattle in neutral.

That said, your description has me thinking your dual mass wheel has failed or, as others have stated, at some time before you owned it, somene replaced the dual mass with a standard flywheel.