I have a 2005 Silverado with the 5.3 and i have a squeal that only happens when in drive or reverse. The sound happens only between 1200-1600 RPMs its sounds like a belt but both are brand new. The sound has been there for 5000 miles at least and hasn't gotten any worst or any better. I wonder if anyone has had this problem or encountered something like this. Any ideas would help. Thanks

 

Welcome to the forum...
It might be worth replacing the tensioner pulley or possible the whole tensioner... don't apply any dressing to the belts, if you detail the engine avoid getting anything slick or slippery on the belt or pulleys....
check for anything that might be contaminating the belt like a leaking AC compressor or oil from a leaking oil pan...
Just some things to check and consider...

 

Take the belt off and see if you sitll have the squeel.