1994 GMC Truck A/C clicking noise

I have had my truck now for 4 years and have never had a problem with the A/C. I am currently in the Air Force and i have just moved down to Florida from Wisconsin. My truck sat for about 2 years when i was in Germany. I drove it it for a month when i was home and my dad would drive it every once in awhile. When i drove it down to Florida i used the A/C almost all the way down with no problem. A couple of days ago the A/C started to have a bad clicking noise. i thought it was just when the A/C recirculation. When the problem is going on and i hit the Rec button i will stop for a few seconds and start again. Then I turn it off and it stops for a few seconds and starts right up. It also does it when i shut the truck down. It doesn't do it all the time just once in awhile. Any soultions you have would be great. Thanks.