I here a high pitched whistling over 50-60mph when the top is down. If a passenger puts his/her hand in front of the hole in the passenger mirror, the sound goes away.

Visually the mirror looks fine, same as driver's side. Anyone have this issue? Any simple fix? Not a big deal, just kind of annoying.

 

Could be the seal around the mirror, where it flexes for impact. Check to make sure the rubber is healthy, and forms a good seal. You can try some Gummipflege, too, which will sort of add some moisture back to the rubber seal.
CATTMAN