Hi all, I have a bit of a head scratcher here. MY 84 300D has a slight vibration at some times under certain conditions. This vibe seems to occur at highway speeds after a few minutes of driving when either coasting (down a hill, or if traffic is slowing up ahead) or when reducing the throttle. It sounds like it's coming from up front but I can't tell. The sound is worse than the vibration, kind of a rhythmic thumping. If you get back in to the throttle, the noise goes away. I am wondering if it's a wheel bearing, since being loaded or unloaded seems to make a difference.

I would suspect a driveshaft problem but in the past 24 months I've replaced the prop shaft itself (driveshafts.com), the flex disks, trans mount, both engine mounts, diff mount, and CV axles. I have newish tires and have had the wheels balanced and rotated several times. The noise and vibe are unchanged. I am wondering if maybe I installed a flex disk backwards or something? All the bolts and nuts are tight. So is the big nut on the prop shaft. With other cars I've had CV axles give me vibe and noise when loaded vs unloaded, so I was hoping that they would cure what ails my car but they did not.

I am beginning to get stumped. Whenever I get up under the car and inspect various components, the suspension is very tight. The PO had the entire front end rebuilt a couple years back. It drives fine. I am thinking of getting my grease gun back from my buddy and greasing the zerk on the new driveshaft - they said to service it at 25k miles and it's been about that many.

Any help would be appreciated.

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1984 300D 193.5k miles
ASTM-certified B100 bio when able