I just acquired a 1973 450SL. It was very sluggish when I got it (not driven much), I have done some tune-up work on it and engine is running pretty peppy now. However, I noticed if I accelerate hard, from a stop, there is a loud thumping sound (thump thump thump...) that comes from under the floor just behind the seats for a few seconds (until I let up a little – then it goes away). This only happens on a foot to floor acceleration and I only noticed it because I was seeing what improvement I had made on the pick-up if the engine. I know what wheel skip is and I am pretty sure it is not that. The suspension is kind of spongy and I wonder if the drive shaft gets torqued when the weight of the car shifts back… Any thoughts on how to isolate and fix?