I bought a '91 200TDI today, after looking at it and test driving it a few times a few weeks ago. On the test drive everything seemed great mechanically, but I did not get to take it on the motorway (although I did test it at speed). On the way home 1.5hrs (London to Salisbury) I would sometimes get a loud roar, usually between 65-70mph, although not exclusively in that range and everything would shake. It didn't always happen and I couldn't figure out what would trigger it, but always if I let off the accelerator or pushed down, the noise and vibrations would clear. If I then let the brought the pedal back to the original position sometimes the noise would come back, sometimes it wouldn't. Any idea what the issue could be?

 

I'd start by inspecting fluid levels and condition. Change them, if you have no previous history. Look for unusual items/material in used fluids...especially trans/gearbox and transfer case and axles. Check u-joints on driveshaft also.