Surprise Diff Noise

Hi, I have a rear end noise in my XJ40… I agree with the Turbine like sound it makes so I guess it is either output shaft or hub bearings. Not sure how I will figure out which. It seems way too loud to be a tyre issue.

But its the history of this that has me perplexed. Car is an 87 3.6 Daimler. We drove it around for a while (say 40 km) and it was quiet and smooth… just a little shocker knocking. So, I replaced shocks and all the shock related bushes. Its non SLS and still has the coil over type setup. Well blow me down if it didn’t start howling straight away… first drive. The noise is definitely speed related and does not vary under brake, under load or even under differing road surfaces… I have the joy of living on gravel roads…So, why on earth would changing the shocks cause this?..and what has it caused? All suggestions (other than painful ones) greatly accepted. Cheers

It could still be tyre noise, which is now evident as a result of the dampers now being stiffer, so causing the tyres to have to absorb more resonant frequency.
Alternatively it could be a bearing became damaged when the work was being performed.

Before condemning the differential change the oil. As the EP properties break down the gear teeth start to rub against each other, instead of floating on an oil wedge.
This is why gear oil goes black and silty.

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If you just did some work, i.e. just changed the shocks and you had no noise before doing that, it seems logical that the noise is from something related to the shock change.
My Ford Explorer boat hauler has a whine in the diff but it isn’t constant and varies in intensity depending on speed, accel/decel etc. Just something to think about.
Also, when you did the shocks, did you remove/refit any drive train stuff?

Hi Larry. No need to remove anything other than shocks and wheels. I did one side at a time so no risk of mixing wheels. I guess its jyst one of those things. Will have to use my stethoscope to figure out where the noise originates. Cheers

Morro …

I think it’s much easier to troubleshoot a noise from the rear end when it’s “speed related”.

Using a straight smooth road accelerate and listen for the noise. If it’s speed related it’s definitely coming from the differential, universal joints, or rear wheel bearings and not from any suspension components.

Now by removing the rear seat (hey it’s only a couple of screws) it’s much easier to determine if the noise is coming from the differential or the outer wheel bearings. A problem inside the differential will result in a whine that comes and goes depending on speed and throttle position or if it’s the output shaft bearings they will produce more of a roar that’s totally speed related.
Because of the design which results in heavy loading on the two output shaft bearings this is a VERY common problem on this car. Good news is this can be repaired without removing the diff.

Seems very strange the noise was only noticeable following the work on the shocks ?
Also strange it ’ does not vary under braking or load’, I’d put the rear end on axle stands and have a friend select drive while you have a look at everything that rotates. Worn output bearings can usually be felt when pulling on the wheel at 12 and 6 o clock position while looking at the drive shaft, any in and out movement will mean replacing them. Have you checked the oil level in the diff ? If you decide to change the diff oil it’s best to make sure you can remove the filler/ level plug BEFORE removing the drain plug. If it hasn’t been removed for a long time it can be a real pain to remove lying on your back, and if you can’t get it out you can’t refill it and you can’t drive the car to a garage with a ramp.

Thanks for the advice. Will follow up. Cheers

Thanks for that. Great advice. We have not touched the diff yet. Only shocks abd brake blead as we are trying to get through a RWC (Mot in the Uk. Warrant of Fitness in NZ. Dont think there is a US equivalent) prior to registration for road use. This noise was a bit of a surprise. We had driven the car about 150 km since buying it with no noise at all. Then… post new shocks and rubber… there it was. So. We push on and await our next surprise. Have to do the AC before summer. Fun fun fun. At least the demister sort of works. Need that for a RWC. Cheers

Ah so!! Car now road legal!! Turns out the noise is the right hand out put shaft bearing… so next time on the hoist, it will be done! cheers

Good to hear you are making progress. The drive shaft is also the top link for the suspension, that could explain why you only noticed a noise after changing the shocks. If you have access to a hoist it makes sense to change the output shaft bearing on both sides, it’s likely they are both the same age and it would be a pia to have the other side start making a noise soon after just doing the one side.

I’ve always thought that is a strange setup, it must impart significant side loads to the differential bearings.

Morro …

Once you remove the two half shafts here’s what’s involved in replacing the output bearings …

The design does impart a side load to the output bearings but they are strong bearings, in most cases usually surviving over 100K miles. My 148K car is still on the original bearings and they are fine. A main reason for early failure is diff oil leaking from the pinion seal or the output bearing seals, once the level is low, the output bearings are starved of lubrication.

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