2003 Chevy Silverado Air Conditioning Problem

2003 Chevy Silverado V8 Two Wheel Drive Automatic
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My A/C unit makes a noise when I select A/C. It sounds as if it's the Baffles or what ever directs the A/C flow is cycling. It Clicks and Clicks and then settles into the A/C blowing only from the Defrost registers even though the A/C controls are set to for the front registers on the Dash.