I've never had this before. At very low speeds, my speedometer has a clicking noise. When the cable is disconnected, no noise. It's only audible at very low speed, say 5mph or lower. At higher speeds it is either not present, or not audible and I've gone out of my way to hear it. The speedo works fine. From what I can tell, it is accurate and it is smooth, no jumping or shaking of the needle.
The noise sounds like it is coming from the gears of the angled drive on the back of the speedo. That's a $40 part which I'd rather avoid if possible. Not that I'm cheap mind you, but I'm sure Triumph has something more fun in mind for my $40.00

Any advice or similar experiences out there?