Knock from the rear - diff?

alexanderfoti

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Hi all

If I chop the throttle, I can feel a knock from the rear of the car.

One when letting go of the throttle, another when putting power back on. Its felt more at speed, but can be felt when driving slowly too.

All the rubbers on the prop are new, mounts etc. I feel like its coming from the diff.

This is with the handbrake on and the gearbox in neutral:


I feel like this is too much play... the rear wheels were held still by the handbrake so all of this is in the diff.
 

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I'd call that nothing - certainly far less than I've had on other vehicles.
 
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alexanderfoti

alexanderfoti

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So possibly the UJ in the prop? Whole exhaust and panels have to go come off for me to check it. Grr.
 

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The ringing noise sounds like something on the prop to me.
 
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alexanderfoti

alexanderfoti

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Must be the diff ringing down the prop as the box is in neutral.
 

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So possibly the UJ in the prop? Whole exhaust and panels have to go come off for me to check it. Grr.

did you do the bolts / wheels up?

mine doesn't make any noise at all (in that regard)
 
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alexanderfoti

alexanderfoti

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It also seemed strange to me. Is there oil in the Diff? If it was the Diff then I would have expected a more 'subdued' noise.
did you do the bolts / wheels up?

mine doesn't make any noise at all (in that regard)

The wheels where on the car. It was in a ready drive state but just lifted off the ground.

It's always done it, ever since I got it. Doesn't seem worse to get worse. Expect when it's warm.

It doesn't make any noise inside the car, but you can feel it in your seat.
 

Botus

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I can't feel any driveline slop / noise in mine.... but I don't have a LSD (you know they get a special oil ?)
 
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alexanderfoti

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I can't feel any driveline slop / noise in mine.... but I don't have a LSD (you know they get a special oil ?)

No LSD on the s class only on the c63.

This is internal play in the Diff only. The drive shafts are held still by the handbrakes and aren't moving. I feel its a bit on the high side. I think if I had play in the UJ I would have more noises etc.
 

davemercedes

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I think it's a "stethoscope" job, Alex that will probably narrow it down to a bearing - it it's always done it but got no worse it's not likely to be anything that serious. I've lost touch now but I used to sell Triumph and Rover Crown Wheel & Pinion sets to a North London axle reconditioning company - I spent an hour one day inn their workshop listening to axle noises on and off the car - for starters, when on the car the noise on the left can be very different from the noise on the right - all down to the "trumpet" effect of the bodywork. But they also showed me how a wonky wheel bearing noise travelled backwards and suggested a dodgy axle.
 
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alexanderfoti

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I agree. I not so worried that its going to fail, but its ****** annoying when It happens. Its quite violent when abruptly releasing the throttle.

I did front subframe mounts, prop rubbers and engine/gearbox mounts, so its likely to be:

Prop UJ
Center support flexing (I cant feel any vertical play when pushing up on the prop)
Diff internal play (I feel like its this)
Rear subframe mounts. (I only did the fronts).

I will check the UJ/center joint next time I am under the car but I won't hold my breath.
 

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ah diff mounts, yes I was told mine need doing (at 55k !!!)

6 hour job and yes if you nail it off the line in first you can get a few noises at the back.... particularly if he traction comes in and you try to fight it

allegedly a special tool allows it to be done far faster...
 
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alexanderfoti

alexanderfoti

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It's funny you mentioned that. The amg has two mounts where others have one. It does seem like lots of work to change them, I think it involves removing the diff.. . Which I am not doing.

I'll do them if I have to drop the diff for some reason.
 
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alexanderfoti

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For comparison, same scenario on the C63:


Seems pretty similar to me, sans ringing prop-shaft, not sure where that is from.

I have noticed one thing on the S class. It gets worse when the car is warm. So its likely to be a rubber component.

Looking at what is mounted, it seems there are 2 big bushes for the diff that mount it to the diff carrier.

Then the diff carrier is rubber mounted into the rear subframe.

Both sets require a LARGE puller to remove them and also require a total removal of the diff to replace.... Not going to happen.
 
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davemercedes

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Supposedly the double mountings on the AMG allow for more aggressive acceleration (which I guess is a practical thing)?
 
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alexanderfoti

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Supposedly the double mountings on the AMG allow for more aggressive acceleration (which I guess is a practical thing)?

Yes I have checked, the non AMG models have only one mount. I would have thought it would destroy a single mount quite quickly!
 

thebiglad

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Hi all

If I chop the throttle, I can feel a knock from the rear of the car.

One when letting go of the throttle, another when putting power back on. Its felt more at speed, but can be felt when driving slowly too.

All the rubbers on the prop are new, mounts etc. I feel like its coming from the diff.

This is with the handbrake on and the gearbox in neutral:


I feel like this is too much play... the rear wheels were held still by the handbrake so all of this is in the diff.
Hi Alexander, I don't yet know anything about Merc vehicles, but on LR and Jaguars, this sort of single thud on and off the throttle was caused by worn bushes in the rear suspension links.
 
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alexanderfoti

alexanderfoti

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Hi Alexander, I don't yet know anything about Merc vehicles, but on LR and Jaguars, this sort of single thud on and off the throttle was caused by worn bushes in the rear suspension links.

Thanks! I haven't checked those actually, will check them.
 


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