Your window glass rubbing on the rubber (not a big deal, but sometimes hard to get to go away). If it's the glass rubbing on the rubber, just try cleaning your windows really well: sometimes this will happen from sticky stuff being on the glass and rubbing. You could also try lubricating the rubber next to the window with some silicone spray.
The glass scraping on metal. Bigger deal, because you'll end up with scrape marks in the glass. Not very common though. To fix this, you'd have to take off the panel on the door and see what's causing the scraping. Edit: based on your picture, this seems like it may be the case. before you take things apart though, it would be good to check for any visible debris you could clean up from the top. This also applies to a small rock getting stuck in the door and scraping the window.
The window mechanism rubbing against itself. Again, you'll have to take off the door panel (not super hard, but harder than cleaning the windows). Then try adding a drop or two of oil in the mechanism and see if it goes away.