Good holiday-decor question — answering as the UphillTech team:
This is likely due to vibration from the motor -- try a hard-service or "fan service" light bulb; LED replacement bulbs may also be a better choice than regular incandescent or CFL bulbs in this application.
Full answer by ThreePhaseEel: https://diy.stackexchange.com/a/63755 (CC BY-SA 4.0).
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