Arduino Simple Task - sweeping a buzzer and led, not working as expected
From Arduino.cc:


  Use of the tone() function will interfere with PWM output on pins 3 and 11 (on boards other than the Mega).


Simple solution is to use a PWM that isn't an output from Timer2 :

Timer      OCxA pin    OCxB pin
  0           6        5
  1           9        10
  2           11       3