Air Noise

I have a 2001 Z71. It has an "air leak sound" coming from under the dash. It is coming from the brake pedal arm that goes through the firewall. The sound starts when you start the truck, and changes pitch when you apply the break to the floor. The strange thing is the noise stops when the engine is under load, ie going up inclines etc. But the noise comes back on level ground or going down hill.

Anyone have any ideas on what it could be and how hard it is to fix?

vacuum line for sure. Get it found because you are leaning the engine out. You also could possibly lose brakes. I think there are AC vacuum lines under there. You could have aleak between booster and master cylinder

I had someone look at the truck today. They said that it was the brake booster. I wanted to replace it my self. From what I can find I need to unbolt the master cylinder, unbolt the 4 bolts on the booster, and replace with the new booster and reattach the master cylinder.
When I unbolt the master cylinder will I have to take the brake lines off, or will it swing out of the way with them still attached? I know that the bolts for the booster are located inside under the dash. Am I missing something, or is their something I need to know before I get started?
Any help or advice would be appreciated.

the master cylinder will not just swing out of the way, but move it gently being careful not to kink lines....lines do not have to be removed, just take care not to bend them alot and not to kink them.

one step you didn't mention so i will, unhook brake booster pushrod from pedal.

Thanks for your help. I replaced the booster this weekend. It was not very difficult. Noise is gone. Thanks again!