Whistling sound on my 1988 3.0

[markfagus]

I am getting a whistling sound from my truck, It comes and goes and at various speeds but I have noticed that when I touch the brake pedal the sound goes away but does come back after I take my foot off the brake.
It does not sound like a pad/rotor noise, sounds more like a vacuum line?

Any idea what can be causing this?

thanks

Mark

 

[GonOvr]

Might have a power brake booster going out. Bad check valve (firewall) on vacuum line or bad vacuum line it's self.