Whistle under dash while breaking

I just purchased an f 250 2001. The noise started yesterday. When I press on the brake pedal while in reverse or park I get a whistling noise. I have checked the source and am not sure if it is where the rod goes through the body or if it is the aftermarket towing controls that are making the noise. ny one have this happen?

There should be a felt muffler where the booster enters the firewall. Get up under there and make sure it did not work it’s way out.

Find the [push rod to the brake cylinder and while pushing the pedal with one hand see if you can feel a draught where the rod enters the bulkhead.

it is common enough if your Vacuum booster has developed a leak.

And, or if ideling with your foot on the brake, the engine stutters, or may increase, depending on whether it has MAP or MAF sensing.

Anyway, all signs of a leaking Vac Booster…

A second to Murphy’s idea.

Another vote for a leaking vacuum booster. Check this out ASAP. You don’t want to discover it has failed completely when you hit the pedal for a panic stop.