I just had a thought, it's possible to completely disable ANC in settings, turning them into "dumb" bluetooth headphones. (Enable "Off Listening Mode" in Airpods Settings and the option will become available in Control Center.) If some of us who are able to replicate this effect consistently could try turning ANC off and seeing if the effect still occurs, that would narrow it down to being feedback related from Transparency/ANC or being something external like back EMF.

I just tested this myself and the two ways that I am able to get consistent squealing (stroking the upper body when in-ear and cupping them in the hand) both fail to replicate when ANC is off. So this does point to a feedback issue.

My other thought is that the APP3 may have microphones located next to the drivers in the ear canal, both for measuring fit, and for the new "own voice amplification" feature that appears in hearing control center if you enable Hearing Assistance. Maybe vibration is leaking through the body to the inner microphone.