Whistling sound from brake pedal

Hi guys

I get a whistling sound from my brake pedal on my 1989 defender. If pressed hard it disappears, but it sounds like some sort of air leak

Is it possible that the brake booster is leaking and if yes, how to fix it?

Could be a brake servo issue.
Do the test, With engine off pump pedal a couple of times, then with foot firm on brake start the engine. If pedal goes down more servo/booster is working.
To check for whistle remove vacuum pipe from servo and try pedal [with engine running] New servo is fix if it is leaking.

The brake pedal lives in a box that is bolted to the bulkhead and sticks out into the engine bay.
Perhaps, in an outpouring of sympathy, you've ran over the referee from the Scotland/Germany game and you've flattened his whistle ?

Or it's what @tottot says ...