Diagnosis
A foreign object is almost certainly stuck between the brush head and the handle, or the brush head itself is defective and internally broken.
How to Fix It
Inspect and clear the connection. Remove the brush head. Check the cavity on the back of the brush head for a broken-off plastic piece or debris. Look inside the handle's collar for any obstructions. Reattach the head firmly with a click. Run the brush without putting it in your mouth—if the noise is gone, you fixed it.
If That Didn't Work
The problem is inside the handle's motor assembly. This is not user-serviceable. If the brush is under warranty (typically 2 years), contact Philips for a replacement handle.
Heads Up
This noise often happens after dropping the brush. The impact can crack the plastic inside the brush head or misalign the motor. Always inspect the brush head first after a drop.