I recently had a chance to look at a problem with the air conditioning in my '98 Silverado. I parked it for the winter and everything was fine when I parked it. I took it out for the first time this summer and the A/C did not blow cold air.
The compressor will engage but only for about 3 seconds at a time. It makes a slight rattling sound, but nothing too major. Air is not cold at all, just the same as having the vent on. I was thinking it may just need to be charged, but I don't know enough about the A/C system to go off my own judgment. Also, from my understanding the compressor should not engage at all if there is no refrigerant.
Sounds like a very common A/C situation. There's a small leak of refrigerant (R-134a) in the system, and you lost most of the charge over the winter months. If the system was completely empty (no pressure at all) the A/C compressor would not engage to prevent damage to the system. However, there's often enough pressure left in the system to engage the compressor for short periods of time, while not enough to cool the air.
You can go to the auto parts store and get a few cans of R-134a refrigerant and an inexpensive injection hose/gauge assembly (often comes as a kit). Then, following the instructions, add enough refrigerant to raise system pressure to the proper level. The only problem with this method is that you still have a leak, and eventually you'll be back where you started.
The other choice is to bring it to a technician with an A/C charging station. They can partially charge the system, locate the leak with a leak detector or system dye test, recover the refrigerant (federal law prohibits releasing refrigerant into the atmosphere), repair the leak and charge the system with the correct amount of refrigerant and refrigerant oil. One method is cheap, and the other a permanent fix. It's your call.