Why would a humming noise be coming from under the hood of a 2003 Jetta only when the AC is on and blowing cold and the car is stopped in traffic if the noise goes away upon acceleration?

The frequency at which the compressor is rotating happens to resonate at the idle speed. You could try spraying some WD-40, or PB Blaster around the pulleys, BUT NOT ON THE BELT.

Hope this helps, if so recommend contributor.