1984 300D Knocking sound from right rear wheel area

At the end of a from a trip of about 4 hours, I noticed a very harsh knocking sound coming from the right rear wheel area of my 1984 300D every time I took off from a stop. It sounded like the axle or wheel would fall off. It stopped after I arrived and parked it over night. At the end of the return trip, it started again and continued every time I took off from a stop. The next evening, I got under the car and checked axles, U-joints, drive shaft carrier bearing, wheel bearings, differential oil level and could find nothing definitive. I took the car out for a drive and it would not make the noise again. I have driven it around town for a week and no noise. I guess it only happens after a road trip of 3 plus hours? Any ideas?
Thanks

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