For Safety Sake, Try This;
1. Find a spot on the road with plenty of room in front of you to accelerate.
2. While Accelerating, press on the brakes.
If the brakes are hard as a rock WHILE you are accelerating, that whistling could be related to a bad check valve in your Braking System.
I was getting a high-pitched whistling sound randomly, while on the road for about a month (but I don't drive much). On one drive, I was accelerating by cruise control, when I needed to brake to avoid rear-ending the car in front of me. The brakes were as hard as a rock. I had both feet on brake and it wouldn't budge. Close to panicking, I FINALLY reached cruise control speed and acceleration let off and I was able to brake before hitting the car in front of me.
I was so freaked out, I just kept pressing the brake, expecting it to work, that I didn't really know what was happening, and how to stop it. In hindsight, I know now to either;
1. Turn off Cruise.
2. Knock it into Neutral (Should turn off cruise as well)
Thought about turning off car even, but too afraid I'd lock steering and loose too much control.
Any way, check valve turned high pitch dog whistle happened to me, and over time the booster became filled with brake fluid rather than air. It took the dealer I HAD gone to, two tries to figure it out. They said they changed (1st) The master cylinder and the booster tank and (2nd attempt) the Check valve and a motor/pump? It was given back with the problem the first time, and I had to take them for a ride to prove it was still there.
I know there could be many reasons for the whistling, but the situation I had was wicked dangerous and scary, so it's worth at least testing out.
Hope it's something simpler or less dangerous, but it's worth the simple test.