You need to figure out exactly what it is to know how serious it is, I agree it's likely a bearing, and I've seen bad bearings go months that way, and also have seen them lock up and destroy spindles the next day. (not to mention what it does to the driver's nerves trying to control the car). If you suspect the front, try jacking each front wheel up off the ground, grasp it at 6:00 and 12:00 (top and bottom) and try to wiggle it. It should be tight, if it has slop, or you can feel a little clunking sensation, it needs to be checked. Also roll the tire by hand and feel for any roughness. I believe you can also check the rears this way too, since these cars use IRS suspension. When checking bearings, any scoring or discoloration means replace them. If they are still smooth without any coloring you can repack them and reassemble them to proper preload. If you dont find anything doing that, look over your flex disks and center support on the driveline, any cracks in the disks, and a lot of loose sloppiness in the support are no-goodniks.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale