Diagnosis
That intense vibration and noise usually means the brush head isn't seated correctly or the internal motor shaft is out of balance because something's stuck on it.
How to Fix It
1) Turn the unit off. 2) Firmly pull the brush head straight off. 3) Look at the metal pin (the drive shaft) inside the neck of the handle. Check for any hair, floss thread, or tiny debris wrapped around it. Use tweezers to carefully pick anything off. 4) Take a new, official Waterpik Sonic-Fusion brush head (the generic ones can cause this) and push it straight onto the shaft until it clicks firmly into place. Test it. The vibration should be back to its normal, firm hum.
If That Didn't Work
If the noise is more of a grinding sound coming from inside the handle, even without a brush head attached, the motor bearings might be failing. That's not a user-serviceable fix.
Heads Up
Always remove the brush head and rinse the drive shaft area after each use. A little bit of hair or gunk caught there is the #1 cause of this noise problem.