I've had mine for a long time, and I've heard it hum twice, and never when I've been using it. I'm not sure it's worth postponing an order for. -- View this message in context: http://elecraft.365791.n2.nabble.com/Re-PKA-500-HUM-tp6835942p6838900.html Sent from the Elecraft mailing list archive at Nabble.com.