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Driveline noise.


dobbo

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I have a 1980 928 millage unknown but showing 108000kms. I changed all the oils soon after buying it (diff oil 75w semi synthetic) and soon after noticed a whirrimg sound while driving at speed, the sound didn't change whether acc or decelerating. When I dropped the diff oil it seemed to have a molybond type additive so I changed the diff oil to 80/90w mineral oil with an additive as well but no change to the noise. Can someone recommend a solution to the noise and also what are the signs of worn torque tube bearings? The drive is smooth but just noisey over 60ks thanks Dobbo.    

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The guy who makes the armour plated TT bearings in the USA says "Noisy TT bearings usually growl at low speeds and change their pitch as the RPMs increase then get drowned out by other noises of the 928 under speed"

What makes you think the diff oil had an additive?
What happens to the sound when you shift into neutral on a downhill run?

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Rear wheel bearings or just noisy rear tyres or both , TT bearings are noisy at idle ( in Park / neutral ) as as you rev the engine the TT bearings change noise( pitch ) with RPM , so in other words ( car stationary ) as you bring the revs up , so does the noise from the TT bearings , because the TT shaft is spinning with the engine at all times unless its a manual trans , with the clutch pedal depressed the T T shaft will slow down & stop , but with Auto Trans is is engaged all the time

Had plenty of Porsche's over the decades with very noisy tyres when they are half worn or more ( there is one brand that real bad for that ) , you would swear blind it was wheel bearings

Very easy to diagnose }

A )  another set of wheels /tyres on the rear for a test drive , if noise goes away ( bingo )its shit tyres , if the noise still there then }

B )  With the car on a hoist  ,remove both rear drive shafts from the car
      Remove the rear brake pads
      Put the wheels back on
      With no back ground noise , spin one back wheel by hand and listen as its slowing , then do the same to the other side & listen
     With perfect rear wheel bearings you will hear NOTHING
     With worn rear wheel bearings you will hear a growl  ( naturally the wheel being on not only helps to spin the hub stub axle in the bearing center but also give a little load to the old wheel bearing/s
    You will often find one side is a bit noisier than the other ( in old age )
     If noisy , replace rear wheel bearings

Note } if its a manual trans ( you do not mention it ) then there is a possibility that the diff pinion bearing/s are damaged , this only happens by neglect because when these old manual transmissions get oil leaks , they are very rarely fixed  & they will keep leaking until they literally run out of oil & this badly damages the very large and very expensive pinion bearings & its COMPLETELY avoidable

Neglect is a very human thing
 

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Thanks for the replies. It is a 3 speed auto. tazzieman the diff oil was black but did not smell burnt so I thought it had a molybond additive. Sounds like TT bearings are ok. Will have to try freewheeling in N and check noise again. Tyres are new Bridgestone Potanzas. Will have to check wheel bearings when possible thanks all for your help Dobbo.

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Check the wheel bearings as I described 

OR

The Bridgestone tyres you have have a noise issue ( a bit unlikely ) but not impossible 

OR

The diff ( center ) carrier bearings are shot ( very unusual on these Auto trans diffs ) but if it has been neglected ( low oil level & filthy oil for a long time ) then its possible

Humans are fantastic at neglect

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  • 4 weeks later...

   Mulsanne Meister you were spot on about the wheel bearings. Removed the RH one today rang local bearing supplier and he says no one local will have any he suggested HI-ROLL bearings in Melbourne. Number on side shield is 531492 any info on where to get two or cost involved would be appreciated. I'll search the internet tonight many thanks Dobbo. 

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