No hum - not enough current to create harm or hum. Sometimes you will feel a tingle when it's on, like the hair standing up on your arm. Another time nothing. I put it on my knees once and had a real aching in my right leg bones which stayed for some hours afterwards. I just never know what is going to happen with my zapper, I notice that it has an acumulative effect and if I use it every day for a week I get die-off symptoms - at least I know it's doing something.