Is there hum or is it hiss and buzz? Is it audible with the volume control turned all the way down?
Wide range spectrum starting at 100Hz, all the way up into the 7-8kHz, constant and not intermittent.
Turning volume or dimension up or down changes the signal somewhat, but not much.
I always have a shorted connector plugged in, otherwise the noise level would be much higher (seems a RR problem, the inputs are not grounded).