Diagnosis
The brush head is not fully seated on the metal drive shaft. A loose connection causes excessive vibration and metal-on-plastic noise.
How to Fix It
1. Firmly pull the brush head straight off the handle.
2. Inspect the metal shaft and the inside of the brush head for debris like old toothpaste or hair.
3. Wipe both clean with a dry cloth.
4. Push a new or the cleaned brush head onto the shaft until it clicks into place. There should be no visible gap.
If That Didn't Work
The internal motor mount may have failed. This requires service. Confirm by running the handle without any brush head attached. If it still vibrates excessively and rattles, the unit is damaged internally.
Heads Up
Always remove and rinse the brush head after use. Buildup in the coupling is the primary cause of poor seating and noise.