Loud rattling noise at highway speeds, oh boy!
 

So I was driving my 1980 300TD along the highway when suddenly I heard a loud rattling noise. I immediately pulled to the shoulder and called a tow truck. Before it arrived though I did some testing to see what I could find out.

1. The noise seems to come from directly underneath and between the two front seats (trans or drive shaft maybe)
2. It makes no noise in first or second gear, not until third gear and then louder the faster you go.
3. Shifting and speed do not seem negatively affected. I only noticed loud noise.
4. It's a distinct rattling noise, not a "grinding"

Any thoughts or impressions? Thanks.

Driveshaft support is my guess.

could be the tailpipe hitting the driveshaft.

So, if it could be the drive shaft support or drive shaft bearing, what can I do to confirm if it is?

Get under the car and try to wiggle the drive shaft. You shouldn't be able to feel any play. If you need a new bearing I should have a NIB bearing at a discount for you-PM me if you need one ;)

Well, the tow truck driver and I both got under there and tried to wiggle it when he had it up on the truck. We didn't really feel any play on it. We did see a rubber boot that seemed loose and damaged. Maybe when the shaft spins fast enough that rubber is slapping against the bottom of the car? I wish it were that simple...

Do you get noise in reverse? I'm wondering if your engine mounts have collapsed.

I do not hear the noise in reverse. I actually backed up for quite a bit to see whether or not I would.

from the 126 FSM
 

I had a banging when my center mount went bad. It quit in reverse or when not under load ie coasting or down hill under light throttle.

Well, I'm afraid I've done about all I can do myself. It's just not safe to jack the car up in my driveway (considerable incline and very bumpy). So, judging but what I described do you think it's safe to drive to a nearby mechanic? Or is that likely to make a problem into a catastrophic problem.

Mystery solved! It turns out the rubber boot on the transmission side of the center support had torn and was flapping around at speed. Cut it off and bam! no more noise. Cheapest fix I think I've ever had on a Mercedes.

So the question is, how essential was that rubber boot? Should I be concerned that whatever it was covering is now exposed? It'd be pretty expensive to pull out the drive shaft just to replace a rubber boot of course...

If the boot was torn, brittle, etc... I would suspect that your center carrier is headed south as well. R&R the drive shaft isn't expensive at all, unless you're paying someone else to do it.