Mirror related whistling sound

When driving I hear a whistling sound - the type of sound that makes you check to see if your windows are all shut, and they were.

I put my hand on the portion of the mirror that I colored in red, and the sound goes away, anyone ever notice this?

(the mirror pic is one I found on eBay, but the mirror I am referring to is the drivers side - didn't want to go outside and take a pic, so borrowed this one). Thanks!

bump in case anyone has any suggestions