My 08 F250, 6.4, with 71,000 miles has found a new noise, of late. The truck starts, runs and drives fine... except, there is a humming noise that can be heard at operating speeds. It is louder/more readily distinguished at a stop, with the foot brake on. The boost gauge only gets to about 10 pounds, (first line), with normal driving and will drop off as I cruise. At idle, it is zero'd out. However, if I stomp on it, everything is forgiven and back to normal. Boost gauge jumps to 23/24 pounds and stay there, as long as I keep my foot in it.
The truck is stock, never had any after market anything and I bought it new.
I'm think the slow speed turbo is out, or has developed a leak. I'd rather it be a hose leak, but, my luck doesn't run that way. Any ideas?

 

had the same noise

I narrowed it down to either the power steering pump or the power steering box.

I added red lubeguard transmission additive (drained enough to add the bottle) to the power steering resivoir and the noise went away.

theres probally some under lying wear going on, but for now, my noise is gone.

 

Hmm, a little dye can't hurt and it may end up saving me some money. Thanks

 

You nailed it, speakerfritz. Power steering pump.