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.

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

My guess is tires, but that's just a guess. If you have any low noise pavement around you can test drive on that (we have that here in Seattle in certain areas, quite remarkable stuff). Of course the DOT would rather spend millions on concrete sound walls than thousands on low noise pavement... not that I am bitter...

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

The rear tires could easily be the issue as stated above...degenerating plies, or even polyester cords can cause this. Any serious previous alignment or balancing problems could have left the rear tires cupped or otherwise unevenly worn which is what you could be hearing and feeling.

Otherwise the hanger bearing is a good place to start, and the other thing that can happen over time is that the driveshaft splines themselves get worn, and have excessive play which generates the sort of slight vibration you might be feeling hearing. The spline/shaft alignment should only be an issue if the driveshaft has been taken apart for some reason.

Thanks for your replies. The rear tires were on the front when there was an alignment problem and could be an issue though I don't feel anything when I rub my hands over them. The transmission mount was replaced because it was broken and I assume could have stressed the center bearing so I will have to get that checked.

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata