I'm not quite sure what you mean by the "auto tranny knocking like crazy". There are 2 flex discs on the W124, 1 front, 1 rear. Usually the front one goes bad before the rear one. A bad flex disc will make noises, though most likely only a single clunk, immediately upon stepping on the gas pedal. The flex disc is designed to do just that, flex. However, over time the repeated forces weaken the rubber disc and loosen the metal bushing within. Don't let this go too long or it could spell serious trouble if the disc were to disintegrate while you're driving. If indeed this is your problem, replace it with one from MercedesShop/Fast Lane. Only $65! Probably an hour to an hour and a half install time.