I agree with the Armorall solution, works great, no more squeak.
It's not necessary to spray it in there, if you look at the rubber piece in the front corner there's only the outer lip that actually contacts the window. Just put some Armorall on a small piece of rag or towel and apply it to the inner surface of that lip (with the window open of course). Wipe off any excess that gets on the outside, and voila! Quiet.