I've often wondered where that Noise comes from..... At Idle it Ding Ding Dings, then Behind that is a very mechanical gear driven whirring Sound, Quite Audible and slightly Disconcerting....Is that the Noise you mean ?? When you rev it, the Noise increases in Freq and dissappears.I think its something to do with the Drive for the Waterpump, The KX500's were'nt designed to Idle like a 4-Stroke and maybe thats Your Big KX saying " Lets go for a Ride !!"
My 97 has made that noise since Birth, and my CR500 makes a Similar
Noise, though a lot Quieter.