Timer Application

I developed a timer application for the smartwatch. You can increment/decrement the time on timer by clicking on the buttons and start it with both buttons pressed. When the timer hits zero, the passive buzzer will beep. However, one thing I noticed is that the passive buzzer will still buzz and make tiny noises even after I stopped it. The Arduino function I am using for the buzzer is tone() and it sends on and off signals to the buzzer in specific frequency. I am not sure what is causing this, so I need to further investigate it.