Question about Philips Sonicare 9900 Prestige

I've had this brush for a few days. When I get to the incisors, especially the upper ones, my whole skull booms in a high-pitched tone, like when a microphone gets too close to a speaker. It's almost unbearable, it gets a bit better when I switch to the lowest intensity but it doesn't go away. What is the reason for this, can I do anything to improve it?