@G- Mini - Basically all rotors will build up a corrosion layer on the rotors and sometimes on the pads depending on how metallic they are. Usually any corrosion on the rotors or pads will dissipate after the initial few applications of the brakes. The poster is stating the noise occurs even after driving for a while, which would tend to have one look at another cause. For the sake of safety and piece of mind, I think it worth having the brakes serviced, which would require the calipers removed, everything cleaned and lubed and everything properly inspected.
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