i noticed a tapping sound coming from my sub the other day, def tech pf15. \njust like someone is tapping on a microphone. \nit is very low but noticeable if the room is quiet. \n\nthe sound is present with\/without source connected, with a different outlet for power, or even knobs\/switches moved around. \nit works fine otherwise.\n\ni'm curious what it is but more in how to correct it.