Hey...my trucks been sitting in the garage for about 4 days or so and i just today took it to work again...about 30-40 min after driving it i start to hear this very loud humming/whistling sound...it almost sounds electronic so i turn off my stereo and subs and its still there....but it still sounds electronic....any ideas? its a 1999 gmc sierra 5.3L with exhaust and modded intake. Oh yea and it only happens at over 60mph and then when u pin her she gets to say 80mph goes away and then comes back at 90mph to 100mph.

 

Maybe try turning the truck off while driving. I have done it a few times to try to hear where squeaks and rattles come from on my truck. Just be sure to put the truck in neutral, and no turns or sudden brake stops ahead.

 

Try tapeing both ends of plastic below your windshield

 

sorry dude i installed an sts kit on your truck, ill come and take it off.