2002 Chevy Blazer Howeling and Whistling under the hood

In the last 2 days my truck has begun whistling & howling when I put in drive and drive. When I sit idling at a light I still hear the howling and almost like it's moaning at me. After about 35 minutes of driving it somewhat subsided to just a low howl but I could still hear it when I was stopping. I'm not very mechanically inclined and don't want anything to happen while I'm driving. I would like a recommendation as to what I should ask my mechanic to check and fix.