Yes, I remember something like that. I thought it was solved afterwards. I do not see reason why sensitivity off or on can make this problem, beside one - if you hit certain PWM % then motor starts to make "some" noises. It happens to me at certain temperature when I adjust RPM to keep lap time. But it is only "other" sound, not any screaming like with free bearing or so. ALex calls it "muses in model
It can happen if you change sensitivity and accidentally hit that PWM %.
I did not hear of see what is exactly happening there, so hard to suggest something, but for example when I started with development, I had model which was improperly trimmed and acceleration of motor caused yaw of fuselage and that confused sensor installed in nose, so at some sensitivity it started wildly oscillate. That is why I had sensor installed back of CG in first tests.
Regarding connections in such configuration - I give very thin (it is "light" grade, it is much thinner than "standard") cables with timers, so maybe you can try standard or thick R/C cables for larger servos. If you think it is problem of grounding, but since it was one battery, it cannot be this problem. On the other hand it can be problem of too long leads, if motor gets command strongly accelerate, it can make voltage drop on long leads and it will push governor to draw even more and it will make even larger voltage drop.
And the last think I have in mind, one of BECs should be off and cable disconnected from connector, or simly cut, with switch off. That switch only makes BEC on or off, ESC is powered independently and all the time under power if battery is connected. If you do not disconnect that red cable, other ESC will feed it backwards by Y cable. If they are both on, it can make teoretically some strange thing (they are switched BECs) but when I tested them they worked normally also both on, however it is not so "technical"
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Heres is the link to my FB page with explanation and that is used on several models (mostly contraprops, so one battery) without problem. I think Jim saw it and I think that is how he wired all together.
https://www.facebook.com/permalink.php?story_fbid=1346574585520642&id=363984487112995