Weird Sound from drivetrain

Hello,

I have ridden about 2000 km with my Canyon Grizl 6 Al 1by Sram Apex 1×12 without any problems. But since a few kilometers I have these strange noises when pedaling. The strange thing is that they don't occur in all of the 12 gears but only in gears 5-9. 

What I have already tried:

– Cleaned the chain in an ultrasonic tank and re-waxed it.

– Dismantled the cassette and cleaned it completely

– Crank pulled, everything cleaned and regreased. The chainring too.

– Dismantled the chain tensioner and cleaned everything.

The crank bearing looked good, the crank swings freely even when the chain is not fitted and no noise can be heard.

I'm getting desperate because I can't get rid of the noise. The strangest thing is that as soon as I shift into gear 10 the noise is completely gone. But it sounds like it's coming from the front and not a problem with the cassette.