I'm having some trouble diagnosing this one. Here are the details:

The noise is a rhythmic hammering that seems to be coming from the trunk area. The noise is noticeably louder under acceleration or deceleration.
The noise is only present when the car is full of luggage AND passengers.
It will only show up if the car has been driven for 30 minutes or more.
The noise is not accompanied by a loss in power or a strong vibration.
The U-joints and flex joint are in decent shape. There appears to be no play in the drive shaft.
Any ideas?

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1978 Ford F-150 351M
2002 Kia Spectra 1.8l 5-sp
1985 Volkswagen Scirocco 8V 1.8l 5sp
1985 MB 300D Turbo California

"Die Natur verabscheut Vakuum"

It is likely that one or more of the half shafts has a bad u-joint. The clues are the sound and the circumstances of operation when the noise occurs. Heavy load changes the angular position of the joint components and gets the balls in a bad spot. This plus the lubricant has warmed up in the 30 minutes of driving and is then not as viscous. You can probably live with it for a while; but if you need to use the car heavily loaded a lot, the offending items will need to be replaced.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

Yep, sounds like a CV joint going out.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

I'll be third on the axle hs.

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1989 300E 144K