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1980 242DL, took the car out for a spin after an engine rebuild and noticed a VERY loud hum, low frequency, coming from under the car when coasting around 40 mph. It sounds like something hits a resonance at that speed, because it fills the cabin with what sounds like a subwoofer on max, VERY loud. Generally, if I coast and keep the car in gear it stays quiet, but if I take it out of gear, the sound really amplifies. I haven't heard it while accelerating, and seems to go away below or above around 40 mph.
At first I thought tires, so tried a new set of those with no changes. When I had the car apart for rebuild, I inspected all the u-joints on the drive shaft and everything felt nice and tight. I also have a brand new center shaft bearing, spring, and rubber housing installed.
Seems like it could either be the rear diff or the drive shaft, but I can't figure out which is more likely to tackle given that it only happens when coasting.
Any advice?
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