GMC Repair: Noisy whirr / Alternator not charging in 2005 GMC Envoy XL (6cyl), gmc envoy xl, gmc envoy

Question
About 1 year ago I had trouble where my battery wasn't charging.  My mechanic (a friend) said I needed a new alternator.  It seemed a little early for a new alternator, but it seemed reasonable.  He installed a new alternator.  About 2 months ago I was driving down the highway and the lights went dim and "battery not charging" light came on.  We got it home and the battery and alternator checked out fine with a volt meter.  Also about this time the truck started making a "whirrling" sound.  This sound gets worse if I step on the accelerator.  The serpentine belt looks like it is in excellent condition and there is no fluid on the belt.  As a matter of fact it looks like it was replaced when the alternator was fixed.  (I can't ask my mechanic since he has since had a stroke).  I am wondering if the serpentine belt could be slipping or something like that where it could occaisionally not be engaging the alternator.  Is there a way to adjust the tension on these engines?  Is that something I can try myself.

Thanks so much for your time.  What you do is great for the profession of the mechanic.  And is a great way to keep cars on the road.

Answer
there is no way to tighten the belt as it is loaded by a tensioner, which could be the source of your whirring noise that or the idler pulley, though it does sound like the alternator is faulty if you have a charging light on and dim lights, I would see if you can have a local parts store check to see if it is putting out to spec.