Posting this in the hope it might help others as it took me a while to sort.
Symptoms were chain running fine initially but after rough downhill sections there would be a rough lumpy feel through the pedals and sometimes a sawing noise. The lumpiness would go after a while but the shifting would still feel not quite right and gears be noisy.
Full suspension bike BTW that I had built myself. Shimano M8000 trigger and derailleur and Shimano SP41 cable.
I tried various things, chain length, different cable routing, ensuring cable had slack around the bottom bracket to cope with suspension compression etc but nothing worked.
The resolution to 90% of the issues was loosening the derailleur clutch and lubing it. 15 minute easy job. I'd read with the clutch on you should be able to push the cage forward and it move smoothly which it does now but didn't. Think it was mainly the tightness as it did have some lube already. Clutch adjusted pretty much as loose as it will go now (I can always tighten if i lose chains but haven't yet). I've read that the XT derailleurs after market can be set too tight with the clutch.
Still had a sawing noise in 2nd / 3rd but no obvious rubbing but setting the b screw looser sorted this. I think it's to do with the chain tension and a longer chain has helped with a similar issue on another bike. However on this bike interestingly a shorter chain with looser b screw worked whereas a link longer chain, necessitating a tighter b screw to tension chain for 11th gear, didn't.
Something else which made the gears a bit rougher and probably contributed to some of the noise / lumpiness was the chain routed under the cage bridge and rubbing on it. The chain should go straight between the jockey wheels and over the bridge. Initially I thought this was the source of all the problems but routing it correctly didn't change the drivetrain that much and the above issues seemed to have a much bigger impact. It was surprising how smooth it could be even when rubbing against the cage bridge and it wasn't obvious the chain was wrongly routed.
The gears run really smooth now , no noise, and remain the same through the ride regardless of terrain. Really glad as it was driving me mad especially every time I saw a review of XT saying it was smooth and silent! 🙂
Hope if you are having similar issues this may help. Some of it seems obvious now but it wasn't at the time.
