They're cheap here, they may not be where you are. By 'cheap,' that's more of a comparison to Harley prices. Several aftermarket companies make OEM style teardrop mirrors. Remember that one side of the Yamaha bikes has a reverse-thread.
Does the mirror vibrate, or does it just make a noise? You could drill a small hole into the housing, and use some low-expansion foam injected to firm-up whatever is in there, rattling. High-expansion foam will probably break the glass as it cures. For the price, I think I'd just buy a new head.