What causes the buzzing noise in my 1997 Mercedes E420 rear view mirror?

The buzzing noise in your Mercedes E420 rearview mirror is likely due to a faulty auto-dimming mechanism. The auto-dimming mirror uses light sensors to darken the mirror when headlights from other vehicles are detected, in order to reduce glare. If the light sensor is malfunctioning or the mirror control unit is faulty, the buzzing noise may occur. To fix the issue, you would likely need to replace the faulty component or the entire rearview mirror assembly. It's recommended to have it checked by a professional to accurately identify the source of the noise and determine the best solution for repair or replacement.