Humming from back when releasing throttle

Hi

I've searched the forum for the issue I experience, although many similar cases I'm not sure if they really describe my issue. I'm beginning to experience some humming from the back of my Freelander TD4 ('04). It's only at speeds above 50 kph, and it's ONLY when I release the throttle. When accelerating the car sounds just as it should. So what can it be when it's only when releasing the throttle? Is it my final drive which is about to break down, some VCU issue or? Advices more than welcome

Best regards

Boelle

Something to try, switch the tyres front/back.
I know someone else who did this and removed the problem.
(this is a VERY temporary fix only - rears really needed replacing)

Out of interest, are your rear tyres wearing unevenly?

Boelle said:

I'm beginning to experience some humming from the back of my Freelander

Click to expand...

I usually find this is bait that I've left in the car for a few days in the sun - squid's the worst.

Just replaced all tyres (to Latitude tour) so not sure switcing them around will make the difference. When you say rear needs replacing, do you mean final drive, or?
I can't remember if there was uneven wear on the old ones. If there was, is the car doomed then?

I'm sure Bukko was just questioning whether the noises could be tyre related - eg pressure, alignment or what ever.

With tyres eliminated as the cause, I think the next place to check would be rear diff mounts - they do perish and need replacing - there's 3 and its the front one that usually goes first. Also the VCU mount bearings perish. These problems are usually the small things not the big items.

Thanks GrumpyGel

Hope your right, as the big things doesn't come cheap you shouldn't by any chance have a link to a guide how to check? Of course look for wear and tear, but could be there was some tricks to it. What about testing the VCU? Should I do that in any case. Sounds like a faulty VCU can send your entire car straight to hell..

By the way, a while ago I checked tyre pressure, because the rear was very noisy. One of the rear wheels had to little pressure, so I reinflated it, and the noise was reduced, but still doesn't sound quite right..

diff pinion bearings is a high possibility if noise is on over run ,it happens on over run as little pinion is now driven by the larger crown wheel which puts more pressure on bearings than under power

Ok. Should I worry about it, or just accept the noise if it's the diff. pinion bearings?

fix it ,worn bearings eventually collapse

You think this could be because a faulty VCU, so that should be changed as well?

Take the prop off, easy to do.
If the humming disappears it isn't necessarily a fault with the prop/VCU but it gives you somewhere to go next.

You keep coming back to it, but there is nothing you've said to indicate that you have a problem with the VCU.

Under inflated tyres are far worse than VCUs stiffening up. It will wear your transmission down to breaking point quite quickly. Lets hope it wasn't like it for to long and hasn't done to much damage. Your humming may or may not be related to it. If I was you, I'd start with the low cost options that are good for general maintenance or diagnostics...

1) Change the oil in your IRD. If the old oil is quite grey, suspect damage to the IRD and possible need for a reconditioned unit at some point. If its not grey, the unit should be healthy. 'Grey' indicates worn down bearings.

2) See if there's a lot of movement in the rear diff, eg push the front of it up and down. If there is movement, replace the mounting bushes. It seams most Freelanders over a certain age have needed these replaced, but my '99 hasn't so can't give specifics on replacement - but from viewing threads on here its usually the front one (of 3) that goes and is the easiest to replace.

3) Take Bukko's advice and remove the props. Its a 30 minute job, just undo the (star) bolts joining them to the IRD & rear diff plus the bolts supporting the VCU and take it away - be careful though its heavy and you don't want it dropping on you or damaging it by over extending the UJs. With the props removed you can still drive the car, plenty are driven permanently like this, and can't really do any more damage to the transmission unless something is on the brink of letting go. With the props off you can check the condition of the VCU bearings which are another item that commonly need replacing and cause noises.

With the props off you can reassess the noises. Likelyhood is they'll stop. Jamesmartin's advice on rear diff pinion bearings sounds good so with low-cost options eliminated will be worth doing/considering. If you don't, at least change the oil. If the sounds persist, I'm sure I've read similar threads on here to yours that has just turned out to be the exhaust rubbing against something (or similar). So check to make sure everything looks OK and the exhaust hasn't shifted or dropped off a mount or something.

I'd also say noises can 'travel' through the car, you think its coming from somewhere, but its actually coming from somewhere else. There's threads on here at the moment where people are getting mechanics stethoscopes to try and confirm where noises are coming from, not sure if this could help to listen out of overrun - but you get my point. If you take your hippo to a garage they may be able to put it on a hoist and listen to the bearings in your diff to confirm they are shot, rather like the MOT bod feels for shot wheel bearings.

Thanks a lot. This walkthrough really helped me a lot. I've planned for oil change on the gearbox anyway, so might just take the IRD as well. Should I consider changing differential oil also?

Regarding my "interest" in the VCU is because I can understand that a faulty one can bring a more than nasty bill with it, and don't know if mine has ever been changed. Bought the car at 106000 Km and has now 205000 on it. But I will go for what you guys have described first.

Thank you so much for your help...

If your car has done 206000 miles on the original VCU it would be the longest surviving VCU is history!
If not, it's still pushing the boundaries of its expected life span.
My 2 previous FLs did 150k and 120k but many go before 70k.
There are always exceptions I guess!

It's not 206.000 miles, but 206.000 km, so around 130k miles for now. But as I'm not entirely sure what the symptoms are when the VCU crashes, mine could have failed, maybe?
Or, I'm lucky to live in Denmark where the wildest terrain we experience is the sidewalk
In the weekend I will change oil on all that needs it, so I will probably get some answers whether the FL is still OK or if I'm up for buying the electric car I'm planning to drive as my next vehicle.

my old car was 170.000 odd miles before the vcu played up

Then I could have more miles in mine
How did you find out it was damaged without damaging IRD and final drive?

From what you have said I would say - rear differential pinion bearings which is a full diff rebuild or an exchange diff unit. As a test, taking the propshaft off should make the noise go away as it unloads the pinion bearings (the flanged differential drive shaft is the pinion shaft, the one that the propshaft is connected to).
You are describing a general crownwheel and pinion noise - worn diff unit.

Could be worth cehcking the UJ joint at the back of the propshaft where it joins the diff. Mine vibrated on de-throttling and I found the joint had all but collapsed.

I will check those too when I change oil a million places