Manual Gearbox noise

[AndriesS]

My 4.8GL has 155000 km on the clock and lately I've noticed a new noise when standing in traffic and when it's in neutral. at this point the internal blower fan speed of 2 and more will mask it but I know it's there. This morning on a smooth piece of tar I could hear it in first as well, any gear above that the tyre etc noises will mask it.

Here is the thing, it's not always there, i.e. about 70% of the time it's there, i.e. when I'm in traffic and number of times it won't be there and then after a gear change it will there.

So , could it be the input shaft bearing, or the bearing between the input and output shaft - difficult to isolate right now and should these gearboxes not last more kms?
What can cause it to be there and then be gone?
How serious is it....

[Peter Connan]

Andries, what does it sound like, and can you notice any difference when aplying the clutch?

[AndriesS]

When I depress the clutch the noise is completely gone , so I would guess it's not the crankshaft bearing. I've not isolated the clutch release bearing completely , i.e. it might make a noise under small load / touching but a complete depressed clutch everything is quiet.

You can hear it's related to the engine speed, sounds like a whor whor noise.

[Peter Connan]

I must admit, I am a little bit stumped.

I guess it isn't the spigot bearing, and it's probably not the clutch (although it might be the release bearing, I have my doubts.

My next candidate would have been the "pilot bearing" in the rear end of the input shaft. This might stil be the culprit, but since it is now audible while driving, it seems a bit unlikely. If it were the bearings on the secondary shaft, I would have thought the noise would be much worse when the gearbox is under load?

My next step would be to jack the rear axle up and place it on jack stands, then listen underneath while somebody "drives" it through the gears, in all gears. If it is the pilot bearing, the frequancy of the noise should change as you work through the gears, and it should be silent in 4th gear as that has a 1:1 ratio.

If it is a secondary shaft bearing, the noise should remain coupled to engine RPm, and as said above I would have thought it would change volume as load is applied/removed?

Off course, it could even be something entirely different vibrating due to harmonics or something too?