Greetings!

It sounds like your noise cancelling headphones, usually with their own power supply or battery, is a slightly different ground from your empeg. If it is annoying, you could use one of the car install noise filters (RCA ended, no need for power or grounding) to give you some isolation. It should be enough to clear it up for you, since it sounds like a very low volume noise. Either that, or run a ground connection from the headphones (or you) to the empeg so that they share ground.