Did you still have this noise when you "slam" the brakes, braking more than just a slight braking?

I noticed with my first car in Canada (Subaru legacy), that I might have heated too much the brakes during winter and with the cold-hot-cold cycle of the brakes, they swirled a little bit. When you slighty touch the brakes, you feel that the contact between the disk and the pads is not even. When you brake more stronger, the braking is almost normal (could still have some bad feelback).
May be you got that this winter, a bad hot-cold cycle of your brakes.
I have been told that it usually will disappear with use but if it is too much swirl, you will have to re-machine the brake disks to even their surface.

I hope this help.