I'm no expert on headsets, having only just started flying, but I do know a bit about acoustics and audio electronics. It sounds like a flaw in the noise-cancelling electronics. But the fact that it does it on different headsets and different aircraft is weird.
Do you notice the squeal from televisions (the old cathode ray sort, not flat-panel ones)? If so, does it sound anything like that?
Are you quite young? Perhaps you've got exceptionally good high-frequency hearing? High-frequency hearing deteriorates with age, from late teens onwards IIRC.
Are there any controls on the headset for the electronics? If so, does the squeal persist at different settings?
The electronics used for active noise cancellation is intrinsically prone to causing a whistle or squeal if it's incorrectly designed or has a manufacturing fault, because it's a feed-back system. Given bose-x's comment about whistle, maybe it's just a design flaw that most customers don't hear because their high-frequency hearing is poorer than yours.
FWIW, I've just bought a Clarity Aloft in-ear set - but I haven't been able to use it yet because of this bloody weather! I'll let you know what I think of it when I've tried it out.