Low frequency rumble

[davestx]

My 1991 Mercedes 420 SEL with 560 motor recently had work done to cure shaking that seemed to come from both the front and back including: new front tires, 4 wheel balance, alignment, center link, guide rod bushings, new shocks, rear flex disc and maybe a few more things that are currently slipping my mind. This week I had new front wheel bearings, a new transmission mount, differential mount and rear flex disc installed. The car feels great compared to how it was. It seems to have a slight low frequency rumble that starts at 60 mph coming from the back. It is not very obtrusive but I do notice it. It does not seem to get worse with speed and the car feels the same or better at 100 mph. It is hard for me to tell if it is new or if I notice it because the steering is now so smooth. In researching this I came across some posts that talk about marking the drive shaft when disassembling it. Does that include flex disc work or just when taking the actual drive shaft apart? My center bearing has not yet been changed on the drive shaft. I will have it checked. Can anyone think of anything else I should check. Thanks.