It could be that you run your motor at its resonantion rpm. Try another prop with different pitch so the motor will run at different rpm. I expect shorter bearing life time.

I'm using a Plettie with 10 poles on the Spin 66 and Igor timer and my timing is set on 15.
The Hertz are set 8 and it runs perfectly, no chirps. ☺
I've used the RPM from 9000 to 10200 with no problems. (Two different props of course. )

Thanks, folks.  I repitched the prop, reduced the target speed by two units on Igor's timer, and the chirping stopped. 

Plettenberg 20-16. 

AXI 2826 has resonation somewhere over 9000 and over 12000 rpm, so best rpm for AXI is somehere at 10 to 11 000 what means prop aproximately 5" (depending on airfoil, diameter etc) that is why we went from old mejzlik 11x5.5 to new 11x5 - and it even give little more power at higher rpm. I changed AXI in my model just now a week ago, because it also did some strange noise in vertical entry of wingover where it accelerates, but I knew bearing was KO after 3 years as it had already visible axial free play, so it was good life time anyway. And yes you are right, it is good to send motor to back to AXI, changing ball bearings on high quality motors is not trivial task, they always rebalange rotor (but I think I got NEW rotor :- ))) ) and they set axial shimming as it should be. We pay usualy ~30 Euro for "renewing", but it is still much less then new motor especailly expensive 2826/13 :- ))

Yes Plettenberg has one extra bearing on other side of rotor, but it does not mean it does not have some resonation rpm. I tried plet. 20-18 with 6 cells and the same prop, it was silent, however at some particular rpm I also hear "some" strange noise. So that large bearing helps, but it is always there.

However if motor loudly screams, then nothing helps only changing bearing and rebalancing - it is not good to fly such motor, it can anytime loose magnets and it can cost model.

I think it is not as bad as with open bell rotor like AXI, when I first time hear that noise I was scared what it was :- ))))))

that was with old APC 12x6 running at 9000 on AXI 2826/12 and guy from model motors (he wanted to see how we use them) told me about those rpm limits ... flying at 10 to 11 000 was his suggestion even before I got that problem firs time ... so obviously I did not follow his suggestion, but then I learned he was right :- ))))))))))

Last night mine went from a chirp to a growl within six flights.  Bearings were replaced with the more expensive high speed ceramic from Boca.  I was warned but they were already purchased.  Howard's Plettenberg is working well.  No sign of any chirp last night.

Hmmmm.

My AXI was running on a cardinal ARF.
Not the stiffest nose.
Should front or back mounting of the motor also have an influence?

Greetings Robert-Jan