This noise only occurs with vehicle speed, raising engine speed while stationary does not reproduce it? Your truck is equipped with a hydraulic brake boost system, meaning the assist comes from power steering hydraulics. See if at all, it can be reproduced when stationary, try with a combination of rpm increases, brake pedal pressure increases, and steering inputs. Also check the power steering fluid level. Your steering at full lock is going to create the greatest pressure increase. I have not seen a vehicle that matches the symptoms you've given. Does it sound like it could possibly be a wind whistle, and by pressing on the brake pedal you are changing the orifice that the wind is coming through? If it only occurs with vehicle speed, I would suspect that that may be the case. Has any mechanical repairs, body work etc, been performed that may contribute to it's relevance?