Here it goes.

It appears to be a knocking noise that you can hear and feel. Definately from the rear of the car. ONLY under acceleration and ONLY between about 17 to 24 mph. Once the car hits 25 or so it disappears as quickly as it came. Under light acceleration it is not as pronounced. Under moderate to heavy it is definately noticeable and annoying.
At highway speeds the car seems normal. This appears ONLY under acceleration and ONLY at the speeds specified. The noise also seems to change pitch slightly just before it goes away.
Definately somewhere in the driveline.

The flex discs seem so be OK. If you grab the driveshaft and axles everything seems to be tight.

Any ideas?????

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

I assume this is from on of the SD's. You have like 20 cars in your sig. Does the sound definitely appear to be coming from mid to rear on the chassis? Sometimes the tranny mount can cause some vibration and sound. Does it make the sound during the entire drive?

Poo Poo on my first post. I should have read more clearly, sorry. With the knocking is there a vibration that is noticable? Is a metal hammer type knocking or rubber mallet type knocking?

I would say more of a metal hammer type knocking than rubber mallet.

__________________
'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

I don't mean to hijack, hope this isn't considered as such, I have noticed that knocking sound too... heavier acceleration = heavier knocking. Mine is a rubber mallet type and occurs under heavier acceleration from a stop it seems heaviest near the torque peak of 2400-2800rpm. I got the worst knocking flooring it off a stop towing a trailer but can get it to occur in a more limited capacity without the trailer... I too would love to know what this is, because sometimes it can be a little scary

Two Thots:

1. the half shaft axles make that kind of noise when they start to go. its random and comes and goes. you can visually inspect the rubber boot on each of the rear axles to see if cracks/holes exists. I have squirted grease in them and taped them up as a stop gap that works for a month or two. when it gets worse, you might back up for a couple hundred feet and see if it minimizes the noise.

2. the rubber boot and bearing on the center drive shaft can also be the culprit, but this thumping is much more relialbe. it thumps near the middle of the car.

my guess is that the 'hammer' sound is the half shaft axles

diesel don

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DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed

Unfortunately there are alot of items that can be noisemakers back there. The reason I ask about knocking tone, (metal hammer or rubber mallet) is that those are major helpers in discovering the culprit. Metal knocking could possibly involve, cv axles, broken sway bar linkage, broken shock absorber, extremely worn or missing bushings on trailing arm, flex disks, universal joint, even missing exhaust hangers. The fact that the knocking persits through a narrow RPM as you suggest would lead me to think something is out of balance. Maybe the main driveshaft itself or one rear tire.

I agree. When the half shafts go out it usually takes a little warming up before you notice it. Mine sounded like they were going to totally fall apart. But only after going about 10 miles.

The center support bracket usually lets you know by a thumping while shifting. Usually when you feather off the accelerator during shifts.

Some good thoughts here. I will have to crawl under the car myself. My axle boots looked pretty good. The only thing that I really want to inspect is the carrier bearing for the driveshaft. Is there one or two under there?

__________________
'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

I just replaced my center bearing/support. That caused a thumping, very clearly only in the center of the car (under the armrest, actually!) Most likely that is Greasy Beast's problem. If that support is shot, you'll be able to move the shaft up and down and all around really easy. In my case, you could see right through it. That's both a simple fix and a real PITA if the flex disc bolts have seized. Oh, and you'll definately need a circlip pliers.

Actually, I hope that is your problem, vwbuge. But, your description doesnt seem to fit; a worn support won't be as predictable, and its not metallic. My guess is an out of balance tire shaking some worn out suspension rubber.

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Paul
2004 E500 4matic; 72,000mi

If I remember correctly on that, there should be one about 18-24 inches back from the tranny. Roughly even with the back of the front door. There should be a universal joint about 6-12 inches past it. If you can move the shaft up and down and sideways by grabbing it at the support, it's in need of replacing.

I second this. Mine looked fine....but were the cause of my knocking/hammering sound on the '85.

Good luck!

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Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013

Hey, does it do it at the same rpm's in different gears, or are you sure its only at those precise speeds? I still think the only thing that would be that speed specific would have to do with an out of balance wheel.

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Paul
2004 E500 4matic; 72,000mi

Very good point. Speed would be tire related, RPM would be drivetrain.

Thanks, Surf and Turf. After I posted, had another thought. the driveshaft is balanced, and on mine, seemed to have little weights attached in certain places. If one of them fell off, or if somehow something got stuck to the shaft, could that unbalance the shaft, causing it to shake, but again only at a specific speed? But then, why would the pitch increase?

__________________
Paul
2004 E500 4matic; 72,000mi