Noisy transmission: gearbox problem?

[Cider Andy]

2006 front wheel drive 5-speed (VXT75) Transit, 135,000 miles. Over a period of time it's developed a noise in the transmission which has (predictably) worsened.

I'm more or less convinced it's the gearbox at fault but thought I'd seek further opinion. Here are the details of the problem:

1. The noise increases with road speed, both in pitch and volume.

2. At idle, in neutral, clutch engaged there's a loud, light rattle which disappears when the clutch is disengaged. This is less noisy when it's warmed up. Increasing engine revs with clutch in or out, there's no discernible noise.

3. When driving this becomes a whine, akin to a dry bearing. It's worse at higher road speeds (and in higher gears). It's noticably worse in fifth gear. Depressing the clutch at high road speed virtually eliminates the noise, irrespective of which (if any) gear is selected.

4. Clutch has a good feel and works perfectly, as does the gearbox, although the gearchange can be a bit notchy, especially when selecting third gear.

5. In normal driving the whine is there under acceleration and deceleration, although it's a very slightly different whine. Feathering the throttle virtually eliminates the noise.

Because of the above, I've discounted wheel bearings but I can't discount the clutch, gearbox or CV joints although some seem unlikely. The one thing every symptom might point to is the gearbox itself.

Any ideas? I'd prefer to diagnose it accurately than the expensive option of elimination by substitution.

[stevieturbo]

Most of what your saying would suggest a gearbox problem.

[Mroad]

As above, sounds like a Gearbox issue, from past experience albeit on a different vehicle, probably a bearing.
I would check the gearbox oil for metal particles and change it to see if it improves, unlikely though if a bearing is on the way out.

[Cider Andy]

Thanks all, that's pretty much what I'mm thinking.