My 7140 is exhibiting a low level hum in several channels with nothing connected other than the speakers.

I had a louder hum with things connected but a coaxial isolation transformer killed most of the noise from the cable satellite box. Now with everything disconnected I am still getting a low level buzz hum in at least 3 channels. The transformer in the unit itself does hum/buzz a little.

Is there anything else I should try or should contact Outlaw about fixing it?