Good point Exo, I guess you could just take the tach signal, smooth it with a small capacitor, have a bleed-off resistor, and set the pair such that when the tach pulses are too far apart, the average voltage will drop blow a threshold point and trigger the alarm. I would think maybe this could be done better with a comparator , using some hysterisis to prevent the alarm from cycling. Unused sections of the comparator could be used to buffer the tach signal, an oscillator ( alarm )/ buffer to drive the PC's built in speaker.