Hello,
Most likely, the drive arm has loosened from the motor shaft, which causes it to bind up, then release on the way down.

You'll need to get to motor, resecure the arm onto the motor and tighten the screw. Applying a little lock-tite to the thread will help to insure in doesn't happen again.



Motor Access (with unit unplugged from power):

1)Lower unit all the way.
2)Remove the rectangular motor cover
3)Remove whatever is mounted just below the motor, e.g. the blower, transition or vent cover. You should now be able to see the drive arm and link arm which are connected together.
4)Disconnect the two arms
5)Remove the 4 screws that secure the motor housing to the unit
6)Lift the motor up, then out towards you.

Flip it around and you'll see the drive arm. If it's loose, reinstall it, but make sure it aligns with the two switches. On the front of the motor, near the 5 o'clock position, there's a metal, spring-loaded tab you can press that will disengage the motor and let the drive arm spin. Press this in and rotate the drive arm to make sure the arm activates both switches.

Once the drive arm is aligned with the switches, secure the drive arm and reinstall the motor into the down draft.

This should take care of it. If not, you should be able to see anything else that's wrong with it while it's out.

Good Luck,
KC