"ticking" is because of either a shorted secondary voltage rail, or simply bad capacitors. when it "ticks" its because the regulation feedback trips and shuts down the PWM controller in most cases, Overvoltage, undervoltage, overcurrent, etc are different conditions that can trip the feedback. Also any faults in the feedback will cause a false-trip.
the ticking is actually the lockout mode on the PWM circuit kicking in, it waits for a time constant capacitor to discharge beyond a point, and the PWM cycle restarts again. once again, if it overshoots or locks out, the process repeats.
Of course, theres also the fact that the PWM oscillator could be running THAT slow because of a failed capacitor, but it isnt common, its more common to hear a LOUD ring in the audible frequency range from a fault in that type of circuit.
