This is my first post so be gentle with me
First of all a huge thank you to all. These forums have been extremely helpful to date and I would have been lost when I was searching for my newly acquired Jag.
I’ve just purchased a beautiful 98 XJR in black. 1 previous owner covered on average 3k a year and had 49k on the clock when I bought it with a full service history. I’m very happy with it so far, but although the mileage is low, she’s still 15 years old and not without her little niggles.
I spent a few years as a mechanic around 10 years ago so I’m fairly handy with the spanners, however I’m a bit confused about a specific noise and I wonder if anyone has experienced the same sort of thing.
At about 55mph there is a resonant whine from the rear of the car. It only happens when you’re on the power so I’m thinking its drivetrain related. If you lift off the throttle and coast the noise stops. If you go over 80mph the noise fades away (this was tested on a private track, honest).
Some other points to note are…

• The noise changes in pitch relative to the speed of the car. It doesn’t get louder, it just raises slightly in pitch between the above mentioned speed range.
• The side the noise comes from is hard to pin down so its either resonating through the car or is the diff. It’s not an offensive noise or even particularly loud but it’s defiantly not something that should be there. There are also no vibrations so I’m confident it’s not something out of balance.
• I don’t think its rear wheel bearings. Both sides have no play at 3 and 9. There is also no detectable roughness in the bearings when I spin the wheels and the noise doesn't change when cornering.
• I’ve done a diff oil change with appropriate grade gear oil to see if the noise changes. It hasn’t made any difference.
• I’ve removed the prop shaft and checked the transmission output shaft bolt. That seemed tight. Also the prop shaft support bearing seems good.
• I’ve checked the tyres by rotating them around the car. No difference. The tyres are “el cheapo” ones but they’re brand new so I’m not changing them until they’re out of tread. It doesn’t sound like road noise though.
• There is play at 12 and 6 on one of the rear wheels. I’m thinking diff output bearings but as you can imagine I’m not planning on spending around £400 on bearings (one for both sides) unless I can confirm this is causing the noise. I’ve put the car on axel stands and driven it up to over 130mph. I can’t hear the same noise when the rear wheels are driven without load. So these bearings are currently a bit of a gamble I’ve also tried a mechanics stethoscope on the diff, no noise when on a ramp, but again I don’t fancy dangling beneath the car whilst someone takes it up to 55mph so I can check it under load.

I’m almost out of ideas now. I’ve got the car booked into a specialist at the end of the month for a full check-up and I’ll mention it to them when I drop the car off, but between now and then if anyone has any ideas I’d love to hear them.
Also if anyone has a rear diff for an X308 XJR for sale I’d be interested in hearing from you. There are a couple on eBay but it’s risky in my eyes. if I spend £300 on a used diff (there are a few on eBay) what’s to say the bearings don’t need replacing in this also!

 

I'm afraid I can't help you, but I applaud your troubleshooting skills and the detail in your post. I would guess the diff or wheel bearings. Do the cheaper one first.

 

Possibly driveshaft center bearing?

 

I had a similar problem with my 98 XJ8L 4.0, it turned out to be the result of the rear diff oil never being checked by either the Jag main dealer it went to under warranty or the independent Jag dealer who subsequently maintained it. The drain plug was done up so tightly and there is so little room between the diff and the subframe that no-one could get it undone. Surprise, surprise, there was no oil at all in the diff when I finally had it changed - it took a four foot bar to undo the pug when the diff was out of the car.

Could well be worth checking before you start incurring significant expense.

 

Someone has previously mentioned using a portable cassette recorder and taping the microphone at various places to narrow down the source of a noise.

These days a digital voice recorder can be bought for very little money.

Or a laptop + usb microphone that you consider expendable.

With the laptop approach and the right software you can even work out what the dominant frequency is on the recording. The pinion side and the wheel side will vary by a factor of three. (3.08 ratio)

 

I checked this when I dropped the prop shaft. Felt very smooth so I doubt this is the cause. That said it's hard to give it a good spin with the support bracket still in place so I haven't ruled this out. When I've had some brave pills I might crawl back under the car with the stethoscope, chock the fronts with my wife's front bumper, jack the rears up and try see if I can get in there for a listen. Not sure wheel chocks will work if the rears are spinning at 55mph+ and I'd rather replace her bumper than my face!

 

Try having someone else drive the car while you crawl around the back seat to track it down.

On a side note, at my airline we have used a hand microphone to track down pressurization leaks around the cockpit windows. You may be able to do something similar with a cell phone.

 

Done a diff oil change as well. The oil that came out was more like treacle in consistency. Pretty clear it was the first time the oil had been changed. Looked clean though.

 

Was trying this today but everyone in Reading town centre decided to go for a drive when my friend was trying to get up to 55mph so I could listen for it. Will give it another try tomorrow.

I was thinking along these lines. And recording music is actually a bit of a hobby of mine so I have all the kit to mic up the whole rear end of the car if I wanted. But beyond a certain speed I think the wind noise would just destroy any sort of usable recording if I did this externally.

There is however a way around this. If I go to Maplin (sort of like Radio Shack for the Americans on the forum) and buy a few piezoelectric disks I should be able to rig something that records the physical vibrations of the component rather than the sound. I can then wire these to each individual part and record the vibration using a multitrack recorder.

This sounds like a royal pain in the ar*e though however is last resort before spending £££.

 

Plums....this approach albeit somewhat more sophisticated is exactly the method Jaguar uses to track down spurious noises. ;o]

These diffs are not known for weird noises or whining unless they have been abused or have very high mileages.

The XJ8 being a very sophisticated car with a rather complex rear end being so well insulated from the body makes it rather difficult to pin down noises.

 

Bondy....I just had a recollection from some time ago about reading a report from Jaguar....in a TSB I think where they found an unusual untraceable noise in the rear turned out to be air pressure related.
I don't have any more details concerning this so, could be pi**ing into the wind ;o)=)
If it was wheel bearing, diff bearing or UJ related, pitch and frequency will alter under load/no load conditions and is usually associated with detectable vibration.