Once the felt in place, you can move the panes forward or back and lift out the exposed bit of felt and put some silicone adhesive in the channel and replace the felt. Move windows to the other position and do the other end of the felt. This way it stays in the track and doesn't slide back and forth. the front window is often "pinged" into permanent position by denting a spot on the side channel to pinch the front pane. I found that the front pane still rattled enough to tick me off, so I used a fingertip to squeeze silicone sealer into the edge between the pane and the frame. Its REAL quiet now.
Glen Byrns