Windows rattleing when rolled around 1/2 way down..

Ive got Auto Windows and when there rolled around 1/2 down or little lower they rattle...So far the only way i can stop this is simply placing my arm agenst it to prevent this..Im afraid if i dont fix this problem it will cause my widow to get loose and maybe jump track...

anyone know what could be causing this or how to stop it?

89 S10(2D) 4x4 4.3TBI (Tahoe Package)

the weather stripping on one or both sides of glass is worn or gone.