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It's a new XT M8100 drivetrain on Hope hub.
It's only in the lowest cassette sprocket, pedalling and back pedalling, chain isn't rubbing against another sprocket and b tension looks good
hub / freehub bearings worn out? is it the smallest or largest cog?
What chain are you using?
. <edit> read the post 1st ya fool
Tectonic shift?
Limit screw need adjusting? It may be just stopping the derailleur from being properly in line at one end of the cassette?
Maybe it's their tummy and they're hungry?
Check mech hanger alignment? Absolutely critical on 12 speed - even on new bikes it sometimes needs a tweak
Some things to check. Sorry if some of these sound obvious, but it's meant to be a comprehensive list not an insult to your spannering! 12 speed is more fussy and prone to very minor adjustments in the following:
1) chain lube (factory lube is more sticky to stop the chain rusting in the packaging rather than lubricating)
2) chainline (is it a boost chainline/chainset on a non-boost frame or vice versa for example?
3) B tension (seems the most obvious from personal experience)
4) chain length too short extending the cage too much (judging from your other post this seems likely - the chain is barely contacting the upper jockey wheel. Always adjust your chain with your wheelbase at its longest point bearing in mind that isn't always fully compressed depending on your suspension travel path).
5) limit screws very slightly out (these are best adjusted without a chain on the bike as the tension of the chain can give a slightly misleading reading)
6) Is the cage clipping the spokes?
7) hanger alignment
8) maybe turn the clutch off and see if that helps (not a long term fix but maybe the clutch is too stiff)?
I know you said it's OK, but very large cogs are particularly sensitive to B tension adjustments. Maybe try a few tweeks either way and ride it just in case. Watch in slow motion up close how the chain leaves the bottom of the largest cog and transfers to the top jockey wheel. It should be smooth, but the 'timing' is very sensitive and the adjustment indicators on the mech don't always cater for deviation in hanger placement and frame geometry.
It took me ages to sort my new build's chain rumbling. Finally it was a combination of needing more B screw and the limit screw was pushing on the mech just a tiny bit too much. There was also the alloy aftermarket jockey wheels but that was entirely my fault.