Driving our son's project 300D the past two days I noticed that when it gets to about 40-50MPH & I back-off the fuel I hear/feel a kerchunk sound from the rear.

I notice it sometimes also when the vehicle is coming to a stop, but not as pronounced, and only occasionally.

Is this most likely ...

Universal
Rear Axle
Real Ball Joint
Transmission
Other

Need to sort if this is likely in an area we already paid to have repaired (rear axle replaced a year ago) or something new ...

Thanks - David

Check the amount of vacuum going to the modulator on the transmission. A small vacuum leak there (or anywhere in the system) can cause something like (or similar) to what you are describing

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2012 Mercedes ML350 Bluetec 91K (hers)
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1983 Mercedes 300D 376K (diesel commuter)

You may have a bit of slop somewhere in the driveline. If you do, you can get a "clunk" in certain shifting conditions, regardless of how well the vacuum system is working. You can test for this one of 2 ways:

1: With the parking brake set and the wheels chocked, put the transmission in Neutral then crawl under the car and try to wiggle the driveshaft back and forth. You'll know immediately if you have enough slop to cause clunks.

2: If you don't like crawling around under the car, you can do a driveway test. Foot HARD on the brakes, put the car in Drive and maintain firm brake pressure. Once the transmission engages, shift directly into Reverse and keep your foot on the brake. If you have enough slop, you'll hear and feel it.

My SDL has a bit of slop in the differential. Not enough to cause problems, but enough to get a clunk every now and then, especially in conditions similar to what you describe. The "clunk" comes from the transmission shifting in such a way that the driveshaft goes from a driving to driven or driven to driving position (acceleration to coast, or coast to acceleration) with very gentle throttle changes.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
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