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Strange Sound From Differential

Henry Boler

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Hi All,


I'm getting an interesting noise on my current differential gearbox. It's a 1971 3.89:1 Triumph GT6 MK3 with Overdrive.


Is there anyone out there who would know what I mean if I said it was 'scissoring' ?


It sounds like hundreds of scissors snipping, it's a sound that I've heard from things before, but have never mentioned. I'm not sure if I can think of a better way to describe it unfortunately.


If that does make sense to anyone, any ideas what might cause it?



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Would you describe it as a very rapid light metalic sound ??


Is your car rotoflex or not ??


Have you noticed any performance issues when the car is being driven ??


Do you hear the sound when the car is stationary and idling at normal revs ??


How certain are you it's from the diff ??


How long has the sound been noticeable ??


The above will help to ID the issue.





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Is it wheel speed or prop frequency if its a rotational noise


Some odd ideas start with wheel nuts, wheel trims , disc pads, bent drive shaft racking the trunion and spring about. Tyre problems, fan belt or fan blades , dynamo pulley split and on and on

Richards questions will point us a bit better


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Yes, possibly a very rapid light metalic sound.

It is a rotoflex car, yes.

No performance issues at all still more than happy to run at high speed, the sound isn't getting any worse.

Only hear the sound when at high speed on the motorway and usually on overrun.

Not 100% certain it is the diff as when under load the sounds from everywhere come out in strange places, but it does sound pretty clear from the diff area.

Sound has been noticeable for a couple of weeks or so.

I might add that the gearbox and overdrive have been changed in that time too so could be related, but I'm not convinced.

The diff was changed about 6 months ago as the old one blew up, this one is absolutely immaculate both inside and out, I rebuilt it, but it is a bit of a howler unfortunately.



Very hard to distinguish speed of the sound as I only get it on overrun and can't really get a good pitch from it. However I might be inclined to say it's wheel speed as I reckon it would be too fast to be clear if it was at prop speed on the motorway, which must be around 6000 revs or something...

It's a definite rear end noise, don't think it'll be anything engine related. No wheel trims on the car, Nuts all tight etc...

I suppose it might be possible to actually record the sound from the car for you to hear.

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I remember a loose front number plate can make a an awfull racket in  the air flow,  


this is sounding more like something obscure rather than a defined mechanical fault 


check you have  not got a supermarket bag trapped underneath  

or the handbrake cable is on the prop   



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I think it may be an idea to raise the rear end and place the axle on stands; in addition support the weight of the vertical links as well  - you will need to raise the vehicle quite high.


Ensure the front wheels are chocked.


Once the vehicle is secure and with a helper, you can run the engine and engage the gears through the rev range taking the revs to about 5K. This may allow the clipping metallic sound to be identified more accurately. Do not forget to take the handbrake off !!!!!!


It's a safe test, just ensure the vehicle is supported well. Even better would be to support the whole car on axle stands so you can check the front as well even though you say it's a rear noise. I find noises can be quite cunning as they seldom want to be detected !!!!!


The bottom line is you want to be able to inspect the car closely whilst it is running through all the gears and rev range. I have done this test on other classics and it's a very good way of determining odd and hard to locate noises.


All the best and good luck.



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Looks like I've found the culprit. Also re running the back wheels at speed, I've done that recently to no avail so went for a drive instead with a mate looking in the back and found the fault.


There was a loose screw on the diff cover and the washer was buzzing about from the diff vibrations, amazing how much noise a little washer can make!

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