P38 - Strange noise on overrun

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kooky_guy

kooky_guy

Well-Known Member
Posts
2,410
Location
Sandhurst, Berkshire
Hi guys,

My P38 has started making an odd droning/whine on overrun. It seems to be related to road speed rather than engine speed and sounds a bit like an old bus decelerating!

No idea where it's coming from and can only be heard with the windows or sunroof open.

Any suggestions as to what it might be? It's only on overrun and the sound is quite hard to describe - seems to be a mixture of high and low pitched whines!

Not much to go on, I know, but I thought I'd put it out there anyway!

Guy
 
front diff would be my first look then rear ,you can help prove it by removing front prop and trying it a short distance down road
 
rewmer said:
I had this, cured with a new set of tyres.
Click to expand...

Really? How bizarre. Tyres are still good though. It definitely sounds mechanical.

Hi Keith. VCU seems to be ok - I could just about turn one of the front wheels last time I tested it and no other symptoms.

Front diff sounds like a possible suspect - there is quite a lot of play in it, both in the o/s driveshaft and the input shaft but I'm not sure what they're supposed to be like! Would removing the front prop actually prove anything in this case? The diff will still be being driven by the wheels on overrun...
 
buts its not trying to transmit any torque to transfer box ,dosent allways work as depends on condition of diff but is usually a good diagnostic tool
 
jamesmartin said:
buts its not trying to transmit any torque to transfer box ,dosent allways work as depends on condition of diff but is usually a good diagnostic tool
Click to expand...

Hmm, good point. Might be worth a try.

I assume lots of rotational movement at the wheels and the input shaft aren't a good thing with a diff?
 
Really? How bizarre. Tyres are still good though. It definitely sounds mechanical.
Click to expand...

So did mine, I checked everything - UJs, diffs , bearings, VCU then gave up. It wasnt till near the next MOT when the tyres where a littl low that I fitted a complete set of Pirellis and it was gone ! I put it down to the pair of directional tyres I had on the back.
 
kooky_guy said:
Really? How bizarre. Tyres are still good though. It definitely sounds mechanical.

Hi Keith. VCU seems to be ok - I could just about turn one of the front wheels last time I tested it and no other symptoms.

Front diff sounds like a possible suspect - there is quite a lot of play in it, both in the o/s driveshaft and the input shaft but I'm not sure what they're supposed to be like! Would removing the front prop actually prove anything in this case? The diff will still be being driven by the wheels on overrun...
Click to expand...

Guy, I suppose you have checked that you have oil in the diff's?
 
another thing worth checking with the tyres is that they have the same depth front in rear. I have seen it many times on other 4x4 systems where it causes issues with the center diff trying to spin at different speeds. It can over heat center lsd (unsure if these boxes have them) also high points on tyre tread will cause noise
 
Grr. This is really starting to bug me now. I can even hear it inside the car with the windows shut now.

Best description I can come up with is it sounds like a BMW M3! (We have one of their offices opposite us and they all seem to make this funny whiny noise as they go past) although mine only does it on overrun.

Couldn't be the transfer box short of oil, could it? I've not noticed any leaks from it but it sounds like it's coming from the middle of the car somewhere. Gearbox was ok last time I checked it. Not actually sure how to check the transfer box, but I assume it means jacking the buggr up and getting oily.

My untechnical checks so far have consisted of checking the temperature of the diff pans after a run and they're barely warm so I assume there's plenty of oil in them! Didn't get a chance to do anything else at the weekend.
 
kooky_guy said:
Grr. This is really starting to bug me now. I can even hear it inside the car with the windows shut now.

Best description I can come up with is it sounds like a BMW M3! (We have one of their offices opposite us and they all seem to make this funny whiny noise as they go past) although mine only does it on overrun.

Couldn't be the transfer box short of oil, could it? I've not noticed any leaks from it but it sounds like it's coming from the middle of the car somewhere. Gearbox was ok last time I checked it. Not actually sure how to check the transfer box, but I assume it means jacking the buggr up and getting oily.

My untechnical checks so far have consisted of checking the temperature of the diff pans after a run and they're barely warm so I assume there's plenty of oil in them! Didn't get a chance to do anything else at the weekend.
Click to expand...

If there is no oil in the diff there's nothing to tranfer the heat to the casing. There is a filler plug on the transfer case, fill until it runs out.
 
Highly likely to be diff pinion bearings. The noise is produced on over run when crown wheel is driving pinion. Sounds rather like a motor winding down. If it was wheel bearings you would hear it all the time.
 
wammers said:
Highly likely to be diff pinion bearings. The noise is produced on over run when crown wheel is driving pinion. Sounds rather like a motor winding down. If it was wheel bearings you would hear it all the time.
Click to expand...

Cheers Wammers, that make sense - it sounds exactly an electric motor winding down! I was thinking that as I heard a train slowing down for Alton station last night! Apologies JM, if that's what you were referring to but I didn't make the connection.

I have had a play with the wheel bearings recently, but they seemed ok with what limited checks I could do.

Are the pinion bearings seviceable/adjustable or is a new diff the only solution? There seem to be quite a few 'recon' ones kicking about on fleabag. Any point in going 4 pin on the front?

As I mentioned before, there seems to be quite a bit of 'slop' in the front axle. Would that be the same cause, or is that more likely to be knackered splines on the drive shafts?

Guy
 
wammers said:
Highly likely to be diff pinion bearings. The noise is produced on over run when crown wheel is driving pinion. Sounds rather like a motor winding down. If it was wheel bearings you would hear it all the time.
Click to expand...

I have this identical sounding noise on my Defender 90 on overrun and from my thread I have decided it's probably input / output bearing on the transfer box that I'll be checking first along with the front diff while I'm at it. Hope you find the problem but sounds like we have the same on different cars. :eek:
 
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