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Discussion Starter · #1 ·
My car is a 250 4matic auto, 2014, 55k miles.


Over the past two months I've noticed symptoms that are getting worse (or I'm becoming more aware of...). When driving at a constant slow/moderate speed (under 25mph) I feel and hear what I can only describe as shunting in the drivetrain. It seems to come from the centre of the car to the rear somewhere - not from the front ahead of the gearbox. The noise/feel is subtle and disappears in the increased road noise at higher speeds. It is linked to road speed and at constant speed is fairly regular, but not totally. It just feels like there's a tiny amount of slack in the drivetrain that is being taken up and then relaxed again, even at steady throttle opening on a flat road. :frown2:



So now the odd thing...
Four days ago, I went in to MB for MOT and mentioned this to them. I asked whether someone could just drive round the block with me, to advise whether there is an issue or whether "they all do that".
My car goes off to the workshop for MOT, and returns two hours later. I'm told the technician took it for a test drive and could see no problem. "Sorry sir, but we're much too busy now for the technician to go out in the car again with you..."
So, somewhat bemused, I drive home thinking "the car will be due a service before Christmas when the warranty runs out, so I'll take it somewhere else".


But on the journey home and in the four days since, there has been no repeat of this shunting issue, None at all. :confused:



So anyone got any ideas on what they did to my car ? I just do not believe it's a simple coincidence.


cheers
 

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Discussion Starter · #2 ·
So, after a long break, an update:

When the car was then subsequently serviced, the 'shunting' was indeed identified. After keeping the car for two days the diagnosis was "it's the tyres - you need new ones". Apparently this conclusion had been reached because they had swapped wheels with a stock car, and my car then failed to recreate the 'shunting'. (It didn't occur to me to ask whether they had tried my wheels on their stock car...)

I didn't believe the tyres were to blame and was not prepared to spend almost £200 per tyre replacing almost new (5000 miles) tyres, only to discover that this had nothing to do with it at all. So I did nothing.
18 months later, the front tyres were getting a little low on tread so I swapped them for more MOE tyres. No difference experienced.
Another 6 months later and I thought I'd bite the bullet and replace the rear tyres, just half-worn.
When I drove away from the tyre depot, the shunting had stopped! I thought it must have been the tyres all along. But no! After a day or so the 'shunting' returned.

I then returned to the MB dealer, and repeated the history of the issue, adding that rear seat passengers could feel the 'shunting/knocking' beneath them. Also I commented that their previous investigation when they swapped wheels and when I later had new rear tyres fitted, appeared to have one thing in common - raising of the car chassis without the suspension being supported resulted in temporary cessation of the symptoms.

So far they've had my car for nearly a week, and in that time have identified that the noise was being caused by the fuel tank (!!??) which they replaced. But no change (I'm not surprised). And now I'm informed they have identified the rear diff as the culprit (and I'm not surprised by that either - as that is essentially where I was pointing them to from my full description when I left the car with them!)

At the moment I'm waiting to hear about the warranty claim and when they can get a replacement part, and when I can expect my car back. If it doesn't go well, expect fireworks.
 

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Discussion Starter · #3 ·
But on the journey home and in the four days since, there has been no repeat of this shunting issue, None at all.
Sorry, should have said that when the 'shunting' stopped after the MOT, it returned after a few days (immediately after my initial post on here about it!) and continued to do it for the next two years.
 
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