Shrill, insistent beeping. My quad set of Hardt Inferno 3000s will, upon running down the timer, losing heat, finishing the preheating cycle, finding themselves with the doors open, or just generally having a bad day, will take off chirping like a flock of crazed chickadees. So naturally, ever sort of similar beeping sound grabs and holds my attention until I’m certain that it’s not them.