Whistle/whine sound in new truck??

Hey guys, just got a 2011 silverado extended cab z71 4x4 last week, love the truck an wanting to keep it in as good of shape as possible. Nice upgrade from my old '04 1500 with a V6 and 2wd. Anyways, I noticed yesterday when I had my windows down that there is a whistle like whine when I take off as well as when in reverse. It almost sounds like a turbo winding on a diesel but not near as loud. I had my buddy drive by an reverse while I was outside of it an i had a hard time telling where it was coming from in the vehicle. It does not make this sound when idling in park or when I give it gas in park which would lead to me believe it's not something with the engine but rather the drive/powertrain. Anyone else heard a sound like this? Just something that is normal? I dont notice any poor performance or issues when driving, just a low volume whistle. It doesn't seem to do it once up an going, only when at very low speeds, that or I just can't hear it with the vehicle moving faster. I'd appreciate any help y'all got. Thanks.

By the way I recorded the sound with my phone an will see if I can find out how to post up a soundfile of what I'm talking about.