I am having a bit of an issue with my pretty new NX Eagle drivetrain. From new, it shifted and behaved perfectly. About 60 (80 miles at the most; I don't keep track) in gear changes were getting a little slower but everything was still running quiet and smooth. In the easiest 3 cassette cogs (50t and down) I get a grinding sound when climbing in those gears especially with high torque on the cranks (like on Goathill for example). What seems odd to me is that the shifting is perfect, I just get this grinding noise. 

Shop says everything appeared to be setup perfectly and have been very helpful. So I took the bike home, meticulously cleaned the drivetrain (as much as I could without taking things apart) and I still get the same noise. I then took the thru axle out and reset the rear wheel and re-installed the thru axle. Seemed that may have helped a little but the noise is still there. I tried a 1/4 turn either direction on the B-screw and that didn't seem to do much to help either. I've read that small adjustments of the B-screw are supposed to have pretty big effects on the performance of Eagle. The shop would like to see the bike again to try and dive deeper into this. 

Should I bring it back to the shop for them to figure out or explore this on my own? Should I just follow the set-up process myself and see if that works? Any suggestions? I do not have the red Sram chain gap tool but will get one to assist in set up if necessary but I also don't have a problem just measuring 15mm for the gap under sag. I have read that this grinding issue can be caused by a worn chainring but this bike is so new and other than some of the black anodizing having worn off it looks brand new. Derailleur and hanger appear straight, axle is straight and tight, pivots should all be torqued to spec. I haven't pulled the cassette or checked the hub that they all seem perfectly fine as well. I've also read that there is more play/flex in the jockey wheels of the NX/GX derailleur and cages, so maybe it's that and it'll just take some fiddling with the B-screw some more.  I will likely just bring it back to have it looked at but if I can simply do this myself I'd rather do it to save the trip. 

Any help here is much appreciated. Thanks.

Thanks I'll check that. Maybe that's it. It almost sounds like either the front chainring, or the large cogs are trying to hold on to the chain too long under higher torque. Quite puzzling. The chain is probably as close to clean as new. Degreaser and then water thru the part tool chain cleaner comes back ran almost clear by the time I was done yesterday.

Thanks. I did check that bolt last night and that is torqued up fine. Unless it's a bad bearing from the factory I don't think it's that either. It's a pretty new bike. The derailleur itself and hanger are straight. I'm guessing it's the B-screw at this point.

Thanks for all the help and suggestions. Turns out the cassette was bad. It was warrantied and so far no problems with the new one.