I have heard a strange loud air pressure noise out of my 00 F350 lately only when your at 1500 rpm but not on the throttle but not off either. My tail pipe ends just behind the cab so the noise is evedent and it was not ther a few thousand miles ago when I bought it. You can hear the turbo whine when your on the throttle and hear it wind down when your not but this is diffrent than the normal turbo noise.Any Ideas?

My brothers '99 F350 would "bypass and dump boost" until the motor got to temp when it got real cold. Or so Ford said.

ITs the Exhaust back pressure valve. Its a butterfly valve in the exhaust after the turbo that closes when the temps are cold to help the engine warm up. Does it quit when the truck warms?? If not try unplugging it and see if your trouble goes away. The plug is at the base of the turbo.