There was a thread about this about 4 years ago, and after having gone through it myself for the last two days, thought I should share my cure:
Unscrew the speedo cable, and fill the void in the knurled end with a light lithium grease, like Lubriplate. When you screw the end back onto the speedo, some of the grease will be forced into the bushing. It might help to have the front wheel off the ground and spinning while you tighten the end. In my case, I rode the bike up and down the street, added another shot of grease, rode the bike again and bingo, all the noise stopped within a block, and the needle was steady instead of jumping up and down.
