I've had the car for about 5 months now, during which time it has developed a low pitched humming sound when driving. In summary :

- Sound is a mixture of "oooo" and "mmmm"
- occurs from 15 to about 40-50 mph, road noise probably drowning out sound after this. Humming gets louder/higher pitched on accelaration up to 40-50, and louder/lower pitched on deceleration
- most noticeable on deceleration, accompanied by slight vibration in footwell/brake pedal area as car comes to a halt
- front wheel bearings replaced
- front and rear brakes overhauled - all calipers/brackets refurbished, new discs/pads at front
- diff fluid replaced
- half shaft u-joints replaced (slight knocking when engaging drive)
- no apparent play in rocking wheels testing for standard bearing wear
- tyres all nearly new (Enduro rotational type). No signs of uneven tyre wear, high spots etc.
- tyres swapped around randomly during an earlier alloy repaint job - no difference
- jurid couplings appear in good condition
- car on ramp running in gear (traction control off) - no noises
- on gradual braking to a halt, in neutral, slight knocking or bumping apparent from rear area after car stops

It has been suggested by a couple of garages that the noise could be tyres/diff output bearings/brakes (discounted by refurb).
When I did the front brakes the hubs, free of the braking gear, could be spun by hand quite freely without apparent friction/noise.

Any thoughts/similar experiences welcome, as I'm a little reluctant to throw (more) money at random parts......

Many Thanks
AndyP

 

You might search some about the differential. I remember some discussion about a year ago (when I got my 2000) of it being one of the usual suspects. Re-reading some of your post, I remember it was about the diff bearings.

 

The diff is a weak spot on the X308's, especially on the XJR as it did not have an uprated unit fitted. So it's possible the output bearings are failing-they take the rotational wear & also the left-right axial wear as they form the upper 'wishbones' to the rear suspension linkage.

It could also be the rear main subframe mounting bushes going soft-these bushes keep the subframe isolated from the chassis. If they weaken & the subframe touches the chassis during acceleration & deceleration, it will transmit any diff noise to the chassis & ruin the car's refinement.

The rear shock absorber upper mounting & lower spherical joints are another spot that can fail, causing clonks at the rear when going over bumps etc...

 

I was fearing you were going to mention the diff !
Mine is the 4.0 litre, which has the same unit as the XJR I believe (3.06:1 ratio I think).
I'll have a crawl underneath at the weekend to inspect the rear A frame bushes.
I have no clunking relating to the rear shocks (yet). I did these on my previous X300 which wasn't too much of a PITA.
Is there a way to test the condition of the output bearings, as replacement involves removing the diff, not to mention the bearings/collars at £250 a side I believe... If it comes to this I can get a fully recon unit for about £700 which may work out a better option.....
Is it worth looking at the propshaft (centre bearing/u joint) /diff pinion also ?

Many Thanks
AndyP

 

Wear in the diff output bearings can be checked by raising the rear of the car off the ground & supporting it on axle stands. Then you grab the rear wheel at the 6 o'clock position & rock it up & down-any play will be obvious & is the either the diff bearings or the driveshaft universal joints.

If the rear wheel bearings themselves were worn out, then the rear wheels can be rocked from left-to-right as well as up & down.

If the rear wheels can only be rocked up & down, but not from left-to-right, then it's probably the diff output bearings that are worn out. Excessive play/wear is an MOT failure point in the UK as the driveshafts form part of the rear suspension-the 'upper wishbones'.

Due the amount of work required to remove the diff, the best bet is to just fit an overhauled or recon unit that has had everything replaced.

 

Well had a look today.
O/S rear wheel : no play at all when rocking. No movement in halfshafts
N/S rear wheel : No movement 9 - 3 oclock rocking. Very slight play 12-6 , but no movement evident from diff area. The movement is very slight and does seem isolated to the hub area - does this mean worn wheel bearing ? If so this would be the complete opposite of where the humming sound seems to be coming from.
I will take to a garage to get the tyres checked out as well.
A frame bushes seem OK but difficult to see properly without dropping the support brackets.
Many Thanks
AndyP

 

I I had a very similar situation where my 1999 Jaguar XJ8 would make sort of a humming sound that starts around 38 mph up until 50 mph under very light acceleration low RPM but once I passed like 55 and higher, the noise was gone… my noise seem to be coming from the center front of the vehicle. I figured it would be the transmission. So I went ahead and changed the transmission fluid and the rear differential fluid and voilà! noise was gone. I hope this helps, but I change the rear differential first and the noise was still there. After changing the transmission fluid. The noise was gone in my experience. It was the transmission fluid that really need to be changed and afterwards the noise was gone.💪

 

Maybe grease the zerk fittings on both side of the diff.