I have sympathy with you Tom and your noisy final drive.
My original final drive had worn out put shaft splines, so I got a spare from Motoworks uk with, as new splines, and had my drive shaft splines renewed at the same time.
Since fitting the new final drive from Motorworks, and I am not attaching blame on them , I have suffered a weird " rubbing" sound from the back of the bike.
The frequency and noise is as if one part of the tyre is rubbing on the swing arm.
The sound from mine is very different to yours, more rubbing than metallic.
I have no play on the swing arm, and a good enough gap between both, so with the noise only there after the second final drive was fitted, then it is from there .
I can hear the noise at low speeds, wearing ear plugs, up to 30 mph or so and after that it lost amongst the general engine and road noise.
The final drive does feel slightly rough and noisy when rotating the back wheel, but that might be my lack of experience with listening to final drive / driveshaft noises.
There is no play on the bearings when pulling / pushing the wheel.
I can hear the noise even when using either front or back brakes....
Someone has attempted to remove the castellated nut on the output shaft , only to peen over all four lugs... nice . I will use the good one from my original unit.
The output shaft seal weeps very slightly I have only topped up the final drive once, and then only a very small amount.
I will try and capture the sound with the bike, VERY CAREFULLY set on the centre stand, and post it if it audible.
I will strip the drive anyway over the xmas period, as I want to swap the output shafts, and fit new bearings and seals all round.
It is typical that the quiet box has the worn splines, and the noisy box has near perfect splines.. sods law I guess. It will be mix and match time over Xmas
Expect a help thread from me at some point.
Bob