2003 Ford Windstar Clicking noise

All of a sudden my minivan started making a clicking noise as if it was trying to eject a cassette or cd. Heater was not turned on, no cassette or cd in. However, when I turned the heater on to the MAX A/C setting and turned the blower on to high, the air that came out from the vents was hot and the clicking stopped. I let it run for a minute or so and turned it off and clicking noise gone for the rest of the day. This has been happening on and off for the last 3 days. Have you any idea what the problem might be?