After my gs has been running for a while and is nice and warm, I sometimes hear a noise at idle that makes the rpm drop about 500rpm. Any ideas what this noise could be?

Describe the noise.

Does it sound like a hiccup and idle 500 lower for a few seconds and the rpm's come back up?

If so, I would say carburetion.

Or, does it sound like a metal-to-metal contact sound, followed by a loss of rpm's? Where does the sound come from? Might be valves. Might be.