Knocking dryer

my maytag dryer makes a loud knocking noise ! what could the problem be and how can I repair it?

remove the tub and replace the bad roller.

Take a look[ATTACH]246[/ATTACH]that roller will develope a flat spot or even freeze up resulting in a banging noise.
a bad roller.jpg (2.9 KB)

a good looking maytag roller[ATTACH]247[/ATTACH]
a good roller.jpg (3.2 KB)