I have a 1998, GS 400. I noticed yesterday when my a/c is running their is a cliking noise coming, without it the engine runs normal. Also I noticed the resovoir cap was opened and it was alomost empty. The a/c cools fine. Please advice.

 

could be the clutch going out.

 

That's how my compressor sounded before it went.
Hook up a set of gauges and you may notice the low side gauge flutter with the noise...

Good luck and hope it turns out ok for you.