A couple of months ago, a small rattle/vibration noise started. It was only quiet at first, and only occurred under load at about 1900rpm. Since then, it has got a lot louder!!! It sounds a lot like leaking exhaust, but is definitely not the exhaust system.
It occurs over the range of ~1900 - 2100rpm, and seems to be coming from the back of the gearbox. It occurs independent of motion (ie car moving/stationary, gearbox loaded/unloaded, transfer case locked/unlocked). A funny thing is that when the vibration noise is worst (you can hold it on ~2000rpm, and it just makes the noise), the speedo jumps around slightly. If I'm stationary, it'll jump up to 20km/hr intermittently. It does the same thing, but jumps a bit less, while I'm driving, across the same rev range.
2 mech's have had a look (without pulling anything apart), with no real diagnosis. One guy thought the speedo sensor (maybe a little idler gear or something??) may be worn and vibrating in the gearbox.

I've just joined AULRO, and had a bit of a look thru the different forums - I couldn't find any other posts for this, soo...... any help would be greatly appreciated!!!!