1994medtec

i have a '94 Silverado, the caulking around the back window (sliding back window) on the passenger side i guess let go and the window has been banging everytime i hit a bump, close the door ect. It started doing this last winter and last spring i put rtv around the window and it seemed to work, untill now. Just wondering if this is a common problem and is there any "right" ways to fix it (because the rtv was obviously not a permanent solution). and its damn *** cold in the truck too now...any help, suggestions, ideas are welcome, thx.