wihining at mid rpms

I have a whine that only happens between 2900 and 3500 rpms. If in park it'll whine if i hit the gas hard enuf to get it to about 4000 rpms. if i gradually rev it up past 3000 it doesn't whine. also on acceleration, if i ease into it its fine, if i get on it a little harder it'll whine til it passes 3500 rpms. with windows up going down the road it sounds like its behind u. windows down it sounds like its next to you. i replaced the tensioner pulley cuz it was bad but the rest of the pulleys seem to be ok. my friend was in front of the truck when i reved it and he couldn't hear it there but i heard it in the truck.:banghead: