I get a chatter sound from my '81 also. It sounds like a subtle diesel knock when cruising down the road at 4000 rpm, like a low-pitch chatter. It disappears when I let off the throttle. My timing belt/pulleys are good and valves are adjusted. Some days are worse than others but never really that loud. I has been like that since I got the bike over 3 yrs ago and has not gotten any worse, so I just live with it. Does this sound similar to your noise Glenn?
I have heard that out of sync carbs could cause something like this. I also run 87 octane instead of 89 and it seems to help, which to me would indicate ignition timing. Could out of sync carbs affect ignition timing?
I just can't help to wonder how flawlessly smooth she would be if I could quiet the knock. Has anybody out there noticed a quieter engine after they had rebuilt their carbs?