Depending on your driving style, cargo weight, towing or not, etc. brake pads go through a lot. You find many times that squeaky or squealing brakes with plenty of meat left on them have glazed. Many things can cause this including not resurfacing or replacing rotors when you change out the brake pads. Overtime sediment and road debris can become imbedded. The pads are what make the noise, but the cause can vary.
Depending on how glazed the pads are, the noise may only happen when the brakes are cold and fade once they have heated OR stay noisy your entire trip.
Going back to the rotors for a moment, slotted options (like those from Stoptech - https://www.buybrakes.com/stoptech/b...igh-carbon-126
) help release heat friction as well as help remove debris from the pad surface.
I think Vk Gopalakrishnan (Mechanical Engineering Design Consultant) had it broken down quite well:
Brake pads glaze mainly under the following conditions.
1. Continuous application of brake, such as during a hill descent, can cause the pad contact surfaces to reach very high temperatures, and result in glazing of the pad surface.
2. Very light application of brakes during most of the time can cause the pad surface to get “polished” to some extent. For a pad to give good braking performance, the friction surface must necessarily have certain amount of wear during braking, so that fresh surface is exposed for subsequent brake applications. When the pad surface gets polished, there will be negligible wear, and the glazed/polished surface will significantly reduce the braking performance. It can also generate squealing noise in some cases.