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in most cases its the rear drum support wheels or the idler pulley causing this.
the best way to tell is take the machine apart and check them, heres a link to a video to show you how. http://www.repairclinic.com/PartDetail/Drum-Roller/12001541/400518?modelNumber=MDE7658AYW#repairHelpVideoTabs when you have it apart spin the motor by hand to make sure its not the issue. if its tight the motor is bad. hope this helps, thanks Bryan