Whistling/Whining noise

I recently traded my 03 Ranger 2WD for a 2000 Sierra XCab SLE 4x4. So far I love it. I have already been to some places here in NH that I would not dare go with a 2wd vehicle. However, I noticed that after buying the truck I hear a whining/whistling noise when going above 15 mph. It only seems to be happening when I touch down on the gas pedal a little. I can't really hear it when I take my foot off or when I give it a lot of gas. My friend said that it is a noise that is often heard on Chevy's and GMC's and that it is a noise from the air in the throttle? Anyone hearing the same noise? Thanks.

Probably nothing but seeing as you bought it used, did you check to see if somebody removed the air filter ?
Some of the higher flow filters such as K&N do make more noise.