The flex disk is about $55.00 or so right here at PartsShop and FastLane. Not difficult to replace at all. More than likely the front flex disk has had it and that is causing your thump. While underneath be sure to check out the rear flex disk too (although normally the fronts wear first).
'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.