I have a 00 Cavalier Convertible that when the windows rattle like something is "loose" Inside the door on both sides. I brought it to a body shop and they said the Channels need to be replaced at a cost of 350 dollars for the front channel. any suggestions would help

Sounds like the part of the regulator that attaches to the window is worn out. Try popping the door panel off and rolling the window down to see if you can find where the rattle is. I had the same problem with my Sunfire but I never got around to fixing it. I just would roll the window all the way down then back up a little until it was just into the sweeps. It held the window steady and stopped the rattle.

theres a little show at the top center of the windows on the inner side of the window tat come out of adjustment sometimes, ive had to shim mine in the past.