Jack one wheel, leaving the other on the ground. Grasp at 9 & 3 and check for ANY play. A "thunk-thunk" means something is loose, and loose will mean vibration. Since it's intermitant, I'd guess the drag link or tie rod ends. Not enough to just look, you must push on them to check. ANY movement sideways at a ball joint in the steering means replacement time.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!