My friend had a similar problem recently, although not the same as yours. You could hear a ton of faint noise through the rear channels at bootup and when music was playing low, but you wouldn't be able to hear when loud.

I took the harness off the sled and noted that the ground pin for the rear channels wasn't clicked all the way into the harness adapter. Clicked it back in and there has been no noise ever since.

Perhaps the same thing is happening to you, but on the empeg's harness plug?