Crunching sound somewhere in drivetrain

Hi!

My first post on this forum. I’ll try to keep it short.

A while ago I was getting some shifting issues and crunching sounds so I checked the drivetrain and both the chain and cassette were worn out. Replaced both, got my derailleur straightened at a bike shop and I was good to go again. Shifting is perfect and everything runs very smoothly, but the crunching is still there! This is not a normal sand crunch or something, because here is the catch: the crunching doesn’t start until I’ve ridden some kilometers, sometimes over 20 km’s of pure silence and then suddenly it starts. It’s a soft rhythmic crunching sound that happens only if I pedal. Also, when I pedal backwards a few times, the crunchie sound is gone for a while again.

In case it’s relevant, my setup:
Trek Procaliber 9.7 (2017)
Shimano M8000 Deore XT 1 x 11 drivetrain
SRAM PC XX1 11 speed chain

Now you might think it’s the chain, but I’ve read about many people having a wonderful experience with this chain on the M8000 1 x 11.

Does anybody have a clue what it could be? There is no way for me to recreate this sound on a bike stand I think, so it’s very hard to diagnose where it comes from exactly.

Oh btw the drivetrain is squeaky clean and freshly waxed (squirt).

Thanks in advance!