Ever early generation Leagcy that I have owned has had this problem. What happens is; over time (and the window rolling up and down so many times) the rubber that is on both sides of the window (by the mirror) becomes a little fatigued - 'stretched out' so to speak.
Follow these steps to reduce this problem.
-Roll down the window completly
-Carefully remove the plastic cover
-In the area where the window rolls up into, squeeze the rubber edges together as best you can so that you refresh the seal.
-While you have the palstic cover off, be sure to check the mounting screws for the mirror for tightness. Believe it or not, if they are loose, it lets the air whistle in and noise as well!
-Replace plastic cover.
Daryl