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.