Why does my car shake when I turn on the ac? - 1

My car shake a bit at idle, but not more then what you might expect from a 2002 16v 4 cylinder. When it's in drive and your not moving it shakes and rattles more. But when turn in the ac it shakes like crazy and makes a loud rattling and chirping sound. To clarify it doesn't do this when the fan is on. It only does it when the AC botton is on. Also it does the same thing but worse when it's in reverse gear. The car runs and drives fine other then what I just said. It's only a problem because I have to turn off the AC and put it in neutral whenever I'm in a drive thru or at a red light to prevent looking like an *** cause my cars obnoxiously rattling and chirping. What could the problem be? Also about how much would it cost to fix? It's a 2002 Ford Focus ZTS auto with 150000 miles on it. It has the 16v Ford Zetec engine.

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