Apologies. I missed that for some reason...
That twang is usually associated with pinion drive flange not being tight enough. If it was loosened a little it wouldn't do it and if it were tighter it wouldn't do it. The giveaway here is that it happened after the pinion seal replacement 😎. There was a bulletin about loctiting the pinion on to the p38 diff on the later td5 defenders due to this kind of noise if i remember.
Sorry JC, the twang was there before the pinion seal was replaced (and I did loctite the flange nut when it went back together)
I can't do much about it until early March so thanks for all your replies and I'll let you know the verdict once it gets pulled apart.
Cheers