Front sensor constant beep when braking ?

Hi

I have similar problem wih my ford galaxy 2011 60 plate while driving suddenly the front sensor came on with the long constant beep and this happeneds everytime i touch or press then brake pedal and its goes on for about 4 seconds and then off and when ok traffic its more annoying i cleaned up the sensorsto make sure nothing is on them and it happened on the hottest day of the year in London please help....also if there is away to switch it off i will appreciate it it...ford galaxy 2011 60 reg plate...please any ideas how to fix the problem thank you....