Ok here's an update, after doing several tests this weekend...
Noise is 100% rev related. The higher the revs, the increase in "frequency" of tapping. I.e. in 2nd gear at the top of the rev range it's a "BABABABABABABABABBABABABABABAB" noise, very loud, almost so rapid that it's about to turn into a continuous whine. In 4th gear it's a more "tap tap tap tap tap" noise, with a very noticeable gap between each tap.
If I rest my foot on the clutch pedal while driving along, the noise continues - I have to press the pedal down far enough to be able to actually change gear, for the noise to stop. If I leave my foot on the pedal fully down, the noise stays away, until I bring the pedal back up again, and it comes back, initially a little louder.
If, while rolling, I take it out of gear and continue to roll, and release the clutch - there is no noise at all. If I then disengage the clutch again, put it back in gear, and slowly lift the clutch pedal, the noise comes back as the clutch engages again.
The sound is barely audible when travelling at the "comfortable" speed for the gear chosen with minimal drive from the engine. The sound is still very faint but a little more audible when pushing harder with the engine. The sound is at its loudest and very audible when engine breaking in a rev range at the top of the given gear, for instance, if I approach a small roundabout, drop down into 2nd, and as I lift the clutch pedal, as it engages and we start to "lean" on the gear at very high revs.
The sound is still present when centre diff locked.
In my opinion, all of this rules out any of the drive/prop shafts, or diffs/transfer box, as all of these rotate at a rate that's relative to the speed of the vehicle, rather than being engine rev related. This noise is either coming from:
- something in the gear box, and that something is rotating regardless of what gear is selected, but isn't rotating if either the clutch is disengaged, or no gear is selected
or
- something clutch related, and that something rotates whenever the clutch is engaged only
Thoughts?