Reading FAN tachometer/RPM on 4 wire fan using arduino
There are no interrupts on pin 9 (unless you are using Pin Change interrupts...?). You have to use pin 2 (interrupt 0), or pin 3 (interrupt 1) (also, on the Mega: 18 (interrupt 5), 19 (interrupt 4), 20 (interrupt 3), and 21 (interrupt 2)).