Depending on the size of your film, you could try either very coarse steel wool or a barbecue grill brush (it will NOT be gentle, so use with caution, and on LARGE film only - one oops and you've completely removed the emulsion from a roll-film negative). Another idea - mix up some dilute ferricyanide bleach, then dip an electric toothbrush (one of the disposable ones you can get at the drugstore) in it, then hold the toothbrush some distance above the negative, and turn it on. Let the bleach sit for as long as it takes to get a result you like, then wash and hypo-clear again. Lots of things you can do.