I've got a new (1 month old) Marantz Professional model PMD 300CP cassette deck with USB. A few days ago I started hearing a faint, almost imperceptible hum when the unit is powered off. To try to isolate the problem I disconnected the RCA cable, still hums. Plugged it in different power receptacles, still hums. All this even when unit is off. I know I should take it to service center while it's still under warranty but what all of a sudden could cause this hum? Thanks for any clues!