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I would check to see if one of the air conditioner hoses is in contact with the air box or any of the ductwork. If so, it could be transmitting a compressor related vibration into these components and creating a harmonic resonance.
If it will make the noise at idle, perhaps you can chase it down by feel or by using a mechanics stethoscope (about $10 at most parts stores) or a long thin screwdriver pressed to your ear (be careful of moving parts!).
the fact that it occurrs only with teh AC on should point you to looking at the compressor mounting bolts for looseness, noisy compressor, or a vibration being carried elsewhere via one of the AC hoses. Also, try listening at other belt driven accessories on the front of the engine; when the compressor is on it puts more of a load on the belt which could be affecting a marginal bearing in some other component that is being operated by the same belt.
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