Metallic whirring sound from the rear when braking

Whenever I brake at low speeds, I'm getting a metallic whirring type sound from what I believe is the passenger rear. At the last oil change, the dealer marked my brakes as being in the yellow in the rear, which would likely require changing at next service. I've never heard this noise from a vehicle before, so it doesn't sound like any rotor grind/squeak I've experienced before.

I don't get the sound right away when braking, it's only at low speeds and doesn't seem tied directly to how much pressure I apply. That is, it doesn't sound worse if I brake harder.

I have a 2016 TRD Sport with about 32k miles. I'm surprised that the rear brakes would require replacing so soon, especially before the front brakes.

I've decided to skip the dealer on this one and am going to my local mechanic this week, just thought I'd try to get some ideas before I go in so I can research if it should be under warranty or not. I know the brake pads wouldn't normally be under warranty.

 

Just updating this in case it comes up in somebody's search down the road. They cleaned out out the rear brakes (mechanic said a full handful of brake dust came out), and also found a heat shield rubbing on the differential. They charged me $40 for the cleaning and bent the heat shield back so it no longer rubs. Sound is gone. I was expecting a triple digit bill for brake issues, guess I lucked out this time.

 

Out of curiosity, do you use your hand brake frequently?

 

For parking? Yes, all the time.