The down-draft kiln vent is widely used on ceramic kilns.
The vent pulls a small amount of air from the kiln during firing. The fumes removed from the kiln are then vented to the outside through a duct. Most vents are placed under the kiln, often between the kiln bottom and the kiln stand.
There are several reasons for the motor to vibrate:
1) The kiln stand is unlevel. This can twist the vent plate and cause vibration. This may be the cause if the vibration begins right after you move the kiln or after you first install the vent.
2) The squirrel cage is dirty, which throws the vent off balance. To clean the cage, disconnect the 4" duct where it attaches to the squirrel cage. Vacuum the dirt from the vent. Do this regularly.
Some people sweep dirt under the kiln while the vent is on. This sucks up the dirt and vents it outside. Avoid this practice, because it will clog the vent. Keep the kiln room clean, especially around the kiln, to lengthen the life of your kiln vent.
3) The kiln fumes eventually rot away the squirrel cage, resulting in vibration. The solution is to rebuild the kiln vent from a parts kit or to replace it.