I don’t know the Aeris, but I do know Ben at Bird is a good egg and a genuinely helpful person. Have you spoken to him?
The Orange Five had a specific problem sounding like this – it was invariably a loose QR. Possibly a dim question, but have you eliminated all of the other possibilities?
Does the problem occur when riding, and if so, both out of the saddle as well as in the saddle, pedaling and coasting?
As with any mysterious creak, it may not be the first thing you suspect.
A few possible things to look at one at a time:
* Rear wheel fastening (although Maxles have pretty much solved this one)
* Seatpost / clamp / saddle
* Oh, and if it’s a dropper, might want to check that too. Try cleaning and regreasing (or if a dropper, applying carbon assembly paste) to fine out if that solves the problem.
* BB
* Cranks
* Pedals
* Shock bushings
* bearings
* Cable outer end caps in stops, or hose guides
In the past, I’ve had the aforementioned Orange Five Creak ™, a front mech, pedals, crankset, seatpost, seatpost clamp, saddle, you name it.
Also try letting all of the air out of the shock (note the pressure down) and see if the problem persists when you cycle the back end with the bike in a stand (obviously, don’t ride it). You could also drop the shock out and see if it’s a bearing problem by cycling (ie moving) the linkage.
Sorry for the length of post…