Let's see if we can figure this one out...... I have a 1985 300D turbo with 249,000 miles on it. About 1.5 hours into any long trip, a weird flapping sound occurs in the rear from what I can make out. Nobody is ever with me to experience it unfortunately. The frequency of the noise seems to change with rpm. If I park it and rev the engine I don't hear anything. If I turn the car off, quickly inspect to make sure I'm not dragging anything, restart, and continue on my way.... the noise is gone and doesn't occur again for another 1.5 hours! Ant suggestions?????
One more thing....... I notice that sometimes when I brake, I hear a thud noise in the rear. I'm assuming it's some sort of bushing after reading through some other threads. Could this be related?

Sounds like the rear axles are showing signs of wear. Check to see if the rubber boots are intact. If they are still ok, you might try sliding the boots off to a side and repacking the joints with fresh grease.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold

Rear axles are my answer, as well. I had the same thing on my car about 50,000 miles ago (right rear axle). It was the boot around the joint getting caught in the gears. I replaced the axle (you can go with new or rebuilt). Now it's happening in the left rear axle. It'll go along fine for a while then start in with the clapping noise. Once I stop and it gets unstuck, it's fine again for a little while then starts clapping again. It only happens on long trips with extended drive periods. Annoying and mysterious, if you aren't sure what the heck it is!

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-- Jason

1995 E320 Wagon
1972 280SE 4.5
1999 Land Rover Discovery Series II

Rubber boots might be torn and as the axle turns causes flapping sound esp at medium speeds. At too high or too low speed it may not happen.

MVK

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One penny saved is three penny earned.

1985 300D Turbo 179,000miles