Screech from front of engine when IDLE in reverse or drive..

Car runs and drive perfectly. No strange noises while in Park or Neutral. However, when I put the car in drive or reverse and am idling (i.e. brake applied), you hear a very, very low pitched screech noise. It's not loud enough for people in traffic to hear, but when you are parked between two cars, it is audible enough to hear. It DOES NOT make noise when you are actually driving -- ONLY when in DRIVE or REVERSE idling.

Have not changed the AC belt before, not sure about pulleys. Basically, I guess I'm looking for a pulley, belt, or tension that ENGAGES (OR REVS HIGHER) when you place the car into DRIVE or REVERSE.

Just replaced the belt tensioner and main drive belt about 3 months ago, if that helps. Any suggestions.. its actually pretty unnoticable for me, but, I figured it could be a problem I should address and not ignore..