Diagnosis
The sanding pad is not flat or the backing pad is damaged. An uneven surface causes excessive orbital vibration.
How to Fix It
Remove the sanding disc. Inspect the rubber backing pad for cuts, gouges, or embedded debris. Clean it thoroughly. Place the sander on a flat surface (like a table) and look under it. The pad should sit perfectly flat. If it's warped or lumpy, the backing pad assembly needs replacement.
If That Didn't Work
The internal bearing for the eccentric drive may be worn. This creates a mechanical imbalance. You'll hear a grinding or rumbling noise. This is a repair for a service center.
Heads Up
Always use the center hole in sandpaper discs. A disc installed off-center destroys pad flatness immediately.