rattle when a/c is turned on

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I have a 2001 right hand drive saturn stationwagon. Has over 150,000 miles. Has been a good vehicle. The past few weeks, starting to hear a rattle (like a screwdriver tapping fast against metal or a pencil tapping against a desk - hard to explain). Sometimes the rattle happens when you first start the car then quietens. When the A/C is turned,the rattle is constant while sitting still. Driving on the road, it does not rattle. Had the A/C checked. A new expansion valve and drier has been installed. Refrigerant placed in A/C. Puts out cold air. Mechanics do not know what is making the sound. An idler pulley was replaced. Checked the motor supports and replaced the struts so car is kept balanced and running smoothly as possible. If a socket or tool has fallen down in the engine area, I cannot find it and it should have dropped out of the car by now. The sound is heard coming from the pulley/belt area. No "service engine lights" have ever come on. Any ideas what could be happening.