Rubbing bits. 12 speed

Have you changed any bits? New chain not running on old chainring or jockey wheels properly for example.

Or that old chestnut of the chain routed round the wrong side of the tab on the derailleur arm, (although that's usually obvious on stand).

Could be so many things, maybe not even the drivetrain itself. Broken axle, collapsed hub/freehub bearings, and BB bearings are some of the things that spring to mind when noises only occur under load.

Or that old chestnut of the chain routed round the wrong side of the tab on the derailleur arm, (although that’s usually obvious on stand).

I believe it's possible to do this and ride 20 miles without really noticing. At least that's what I'm told, obviously it's not from personal experience. 🙂

I believe it’s possible to do this and ride 20 miles without really noticing.

No, I can't believe anyone would be such a daftie. 😜

FS bike with upper guide - chain clears guide in stand, but not at sag point?

Sounds very likely to the the chain routing through the derailleur. On M8000 11sp I ran in like that for 100s of km.

It's not the chain routing through the mech, cheers though

I misread the title, I thought you were overexcited by your new Eagle groupset......

Found a loose pivot bolt and have played more with b tension. Better but still not great. May just go back to 10 speed

Could be chainring wear.
It takes very little wear on 12 speed chainrings for the chain to not release from the teeth on the bottom run under power.
You can check by having the brakes on, driveside crank at 3 o'clock, all your weight on the pedal and reach down and pull the chain away from the bottom run, if it sticks slightly, that's your noise.
I've had two new chainrings do this in under 3 months each with new chains.

On my Shimano 12 speed stuff, sometimes the chain comes off the bottom jockey wheel and gets dragged through the gap between the jockey wheel and the cage.

I find anything above 10 speed is so much more finicky to set up and more sensitive to any knocks or adjustments.

Give me a ten speed cassette with a wide range. Not fussed about the bigger jumps when off road or going down hill.

When you say you've played with the B tension. Have you used the appropriate tool (SRAM) / guideline on inside of cage (Shimano) to set it at sag? It really is that fussy.

If it's SRAM, is the chain meshing correctly with the narrow wide jockey wheel?

If it's all setup right I would be putting it down to chainring/chainline. Does it happen in all gears or just the extremes? 12 speed setups can introduce some pretty crazy chain angles which can cause noise when meshing with the narrow wide chainring.

I had an issue like this driving me nuts for ages, turned out the q factor of my cranks wasn't correct so every time I put power into the left crank it ever so slightly contacted the chainstay and gave me a very weird "knock" feeling/noise up through the frame.

Frame flexing under power and tyre rubbing on chainstay?

will try the chainring thing ta

While I've found that 12sp outlasts pretty much everything I used before, I have found that jockey wheel wear is higher than before, and I check them more frequently than I used to

The other one to check is if you have a quick link on the chain, whether it's installed correctly. I had a annoying vibration (that sounded like your Brrr Brrr noise) that only really affected the 11T and 12T on the cassette, and eventually tracked it down to the quick link being upside down...

I had a very similar thing on my previous bike, running GX Eagle. Turned out to be the bastard chainring. A new one – steel, not alloy – totally fixed it.

I had something similar, and it was my front top chain device rubbing against the chain when the chain was under tension. Have you ruled that out?