One of the guys at the office just had similar symptoms on his 450SLC. It was the bendix not engaging, actually I think it was trying to engage but was hanging up. The bendix had been pulled off several times and lubed, which help for a short period before hanging again. Since this was still the original starter, he opted to replace the complete starter rather than just the bendix.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.