Whistle/squeal From Under Dash On Firm Braking

[Scooby]

Guys;

When I brake firmly a fairly loud whistle/squeal can be heard briefly coming from the brake booster. Stops when you push the pedal past a point. Car is a '92 32 GTR.

Doesn't have to be super hard stop ie ABS territory, and I can make it happen when stationery by pushing the pedal past a point.

Sounds a bit like a vacuum leak but I've checked the lines to the booster and they seem fine. Booster seems fine too. Mcyl is new and the noise is not coming from the calipers or tyres.

Any ideas? It's not something I've ever heard in another car. And it's not a canary under the dash!

Thanks for any feedback.

[Scooby]

Anyone?