it was cracked gear on mine. these motors are very simple and reliable. but there is a large gear with a brass shaft pressed into it that is suspect to crack. i took mine apart and used some epoxy to glue it back together. before you take it apart, mark the position of the lever to the brass shaft and the large gear to the shaft. there are three motors on the heater box and they are not the same. while you have your heater box out, you nay want to check the levers that help close the doors. two of mine were cracked and i had to glue those as well since they are no longer available.