The source of rear end bumps and knocks is not easy to pin down...might be the exhaust, the spare loose in the trunk, or the rear axle mount.
A support stalk comes up from the rear axle ending in the rear rubber mount. The mount is accessible under a plastic cover in the front center of the trunk. A dried up mount transmits knocking sounds from road bumps.
A new mount is avail from MB and re-sellers for about $60...replacement is a do-it-yourself 15 minute job involving just a few bolts.
A new mount did wonders to relieve most of the myserious knocks from the rear of my car...
Richard M
'71 280 SL, Tobacco Brown, Cognac interior, Tan top, Jump Seat, 80K orig miles, NYC.