Brake dust can cause asymmetry in light brake application. But generally the brake chatter bars are the point of origin. They are designed to "chatter", which usually turns into a squeal if ignored. Hearing this means the manufacturer has deemed the brake pads worthy of replacement immediately or in the near future.
Disc brakes can squeal even if they're brand new. There are ways to fix it, but noise alone is not necessarily an indicator they need to be replaced or that they're not working. It's just annoying.
Ya, if you don't turn the rotors then the new break pads will squeal. Turning the rotors may increase the life of the new pads slightly but hell man, taking them off and taking them to someone with a lathe is a pain in the ass in my opinion.
I'm sure there's other causes but this one I'm personally familiar with.
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