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Another Noisy Diff.


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Hello everyone, just wondering if any one can give me some advice on a rear end noise on my Gt6 Mk1. 

The car originally came with a very whiney 3:27 diff which I replaced with a spare 3.89 diff that I had lying around in the back of the garage. No longer did I have a whine, but a rhythmic thudding noise coming from the rear end, as I had some issues with brake pipes fowling a swing spring conversion and the spring was found to be no longer springy (i.e flat) I decided to do a 'quick' rear end suspension rebuild, new brakes, UJ's, new driveshaft, new rear trunnions, bearings, spring etc, etc. taking about two months of Sundays in the end. The first drive back on the road this week and the Thudding noise was still there :( , i'm assuming as no part was left unchecked or renewed that its the diff!


Its a thudding noise, like a muffled knock, that gets faster and louder with acceleration, and when I turn left goes quiet for a bit and then resumes, but not always.


I'm thinking the half shaft bearings, but maybe its something else, any ideas greatly appreciated.





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Yer bearing wont mek a   thudding noise,

they will mek a grinding noise, or a  grating noise.


Have you checked the tightness of the body t,chassis bolts at rear.

could be the end sliding spline joint,  { wen was last time ye greased it } 

as wen splines wear, they move sideways, and its a thuddy clonk,


esp on slow speeds an on the overun in top gear.

Get odd of it, an waggle it up / doon,  If its worn, it,ll move.,


other thing can be worn UJ,s on back oft prop.

butt, this is generally a clonk, no a thump.


It could also be a shocker,  Ive had cars in that ev a clonk coming frae the shocker.

internally worn oot, an clonks for first bit of movement,

increases wid speed,or bumps as speed rises. as its working faster.


Check yer sliding spline first.

Diff no thump, but clonk an whine



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One common denominator is the tyres

if the banding has a problem it can look nice and round to the eye but when driving the

tread takes on a lobed effect causes chaos, need someone to look whilst you drive past or get followed

To see if it rolls correctly or flops the tread or goes wonky

sorry cant describe this in words



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Have you checked u-joints, which tend to give more the kind of sound you describe? Could be that the old whiny diff. masked that sound?


And even less likely, but possible, is a front wheel bearing. I had similar noises (not so much the "thudding" part, but yes to the part about turning changing or temporarily quieting or exaggerating the noise) on a Herald 1200 that turned out to be bad LF wheel bearings and, ultimately a bad hub as well (somehow worn to the point where the bearing races would not stay put).

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Hi Hagg

Thudding is usually uj's not tight enough

Were the uj joints set tight, With oversized circlips?

You should be able to stand a driveshaft up on it's flange without it falling over, got to be that tight!

Also check the uj cups are not turning in the yokes or flanges

I had the same problem with my spitfire cured with a new driveshaft and uj


Hope this helps

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Hi and thanks for all your responses, lots to consider.


I originally suspected the UJ's which is why I decide to strip the rear suspension and rebuild with new, but the noise was exactly the same after the rebuild so by means of elimination I don't think its coming from them. They were stiff on the bench and they did stand up on the flange.


Front wheel bearings were checked and one replaced last year, the noise i'm pretty sure is from the rear.


Tyres are new with only a few hundred miles on them, balanced & fitted in November.


Rear body to Chassis bolts tightened, well the ones around the diff anyhow.


This only leaves the prop shaft or shock absorbers, the prop had new UJ's last year when I did the overdrive conversion but I would say I don't think i checked the sliding spine joint, might be worth a look at.


Shock absorbers came with the car and are Spax adjustables, anyone got any ideas how you check them?




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Hagg aint checked the sliding joint  Pete.

it really does clonk at low revs in top an on overun.


Note, its a clonk, no a thump,

there is a difference,


is it deffo a thump, as in thumpin something wid a soft hammer

or is a clonk.


it will help diagnose alott

butt loot is still on sliding spline


OR, both diff have  massive play in the planets

this combined wid the spline wear, is realy bad at low revs,overun


So,mebe bothh diffs ev got play in the planets,

moer than likely in my findings



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Hi All, thanks for all your ideas, gonna try and get it on a ramp at the weekend and do some wiggling of bits, its definitely a muffled clonk, what I would call a thud or a thump. Not a loud clonk. I even sound insulated the boot area and it has made no difference, I'm thinking prop, but will check tyres, drums and rear hub stud assembly as they can sometimes get bent during removal.




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Look at the drive shaft , is it bent, this can oscilate the trunnion and the spring eye does a lot of

unwanted movements.


think this is an outside the box problem

props goes at engine speeds and wheels 4 times slower that should point any transmision vibes

is is vava room or round and round we go


the only knock from a diff is loose crownwheel bolts catching the bearing caps, or a really naf tooth failure


Drive shafts uj are notorious for knocking but its a a knock or a clicking, does change on cornering loads as you have noticed.


if you get her up and can replicate the noise use the old screwdriver trick as a stethoscope

to home in on the epicentre


but hanging drive shafts can foul on the chassis ...noisy



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  • 1 month later...

Hello again, finally managed to take a look today, jacked the car up, checked rear hubs & back plates which all seemed fine, but what I did notice was on the passenger side once every full rotation of the wheel it did a tiny little jump, checking the drivers side it did a much bigger jump on rotation, which led me to believe, this could be my problem. The passenger side is fitted with the new driveshaft, just wondering if anyone might know if this is bent driveshaft or something in the diff. To be honest the shaft looks pretty straight to me.




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In almost all diff I tek apart,

the planet thrusts are moer worn on one side than the other


must be something t,doo wid ..round a bouts,  !!!


If ye get hold of the diff input shaft

an twist it back,n forads, then the play that will be there,

will most likely be the planets, with worn away thrusts


this caused by olde oil, not changed often,, all gunge going aroond, wearing things away,   and round a bouts int road.

straight line driving does not effect planets.

Did ye check the sliding spline, for wear, up and doon movements,

also side to side as well


wid cars age, it,ll be both thats causing yer clonk, as said earlier on.



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