2003 F659GS This began last week all of a sudden on the way home. Vibrating (in a way more than usual) at high speed 4th & 5th gear, enough to make teeth chatter and give a whistled note a strong vibrato. Intolerable in 5th, bad, but tolerable in 4th near 60-70mph. Feels like the bike is convulsing / having a seizure (not seizing, the oil is good) I know you can't diagnose it over the internet, just asking if anyone might have had similar issue and what they did? Checked all bolts, they are tight. Could it only be an unbalanced rear tire? Bearing shot? Sparkplug needing change???