oh yes, without a doubt, I'd noticed that in some cases, and eventually, if/when they 'crack', it's just a tiny sliver of a hairline crack in one which lets air

oh yes, without a doubt, I'd noticed that in some cases, and eventually, if/when they 'crack', it's just a tiny sliver of a hairline crack in one which lets air in and produces that same loud feedback squeal. ouch!