Slowing down, I hear a high pitch whistling while braking starting around 10 or 15 mph, to zero. Just creeping in idle, if I brake, the whistle comes, reliably. Can't hear anything at moderate or high speeds.
I've had the car for about 40k miles, the only brake work I've done is one new RR caliper and pads, which was sticking when I got the car. Brakes seem to be working like always except for the whistling. What should I be looking for?

 

Some brake pads come with a metal tab that starts to rub the rotor when the pad is worn down. I guess the sound they make could be described as "whistling." You should check the pad thickness for all of your brakes anyway. There is a phenomena of "brake squeal" that can be remedied with a bit of lubricant applied to the right places.

https://www.lesschwab.com/article/br...and-grind.html

 

RR rotor down to 20mm, pad is almost invisible. One new rotor $25 and 4 wheels pads $40, coming next week from Rockauto. Shipping was $30 and takes over a week!?!
Still half the price of Autozone. I wonder what the shop would charge?

 

Turns out 20mm is fine for rear rotor. Replaced rear pads, whistling gone.
I think the problem was the "loaded" caliper I got at NAPA 40,000 miles ago. One pad was all wore down and the slider pins were dry and rusty.. I guess the two cents saved not greasing them adds up to a lot of Yuan...