Does the rattle go away when you put pressure on the clutch pedal? If so I'd place my bet on the throw-out bearing. Not what I'd call an expensive part, but you have to remove half of the drive train (drive shaft and tranny) to get at it. Not sure if the MB manual shifts have pilot bushings, but if you get into it that far, you might as well replace the pilot bushing while you're in there.
73 280 SEL 4.5