97 Ford f150 makes whistling sound when I press the brake pedal. That's the only time I hear the noise. Also noise is only heard in the cabin not on the outside. Any advice?

 

I had that happen and it was the brake booster with a leaky diaphram. You can check by pumping the brakes after around 15 minutes after shutting down the engine. You should get 3-5 power assisted pumps before the pedal gets hard. If not, check the one way vacuum check valve which is plugged into the booster. If it is ok, you likely have a bad booster which is behind the master cylinder.