So, after trying to figure out what it could be without tearing the plane apart, I recalled an anomaly I "fixed" after my last flight before the problem was discovered: a loose pilot-headset jack. That is, the outside nut was a bit loose and I tightened it. Probably in the process I rotated the jack a wee bit. So today I loosened that nut and rotated the jack CCW and viola! the squeal went away. So a bit of rotation probably cause a contact between the pilot and copilot output jacks - in all likelihood a ground loop (electrical, not Pitts-special style
). I found by wiggling this jack's position a bit I could induce the squeal to come and go. I (carefully) this time retightened that formerly loose nut after twisting the jack a bit further CCW and all is normal now.