Ive got this as well, only does it when ive driven it and getting out of the car. checking tonight its wet but no noise. Could be the seal expanding after the window drops.
mine is more like air not water though, like the cabin is airtight which i know its not.
different note, love the little window, seem to havee it dropped a lot to get fresh air better than the main window. also the visibility is a massive improvement.
like it as well that i can drop the window quick when getting in the car, close the door and it auto closes.