new panasonic deck

well i finally installed it, but im still having one problem. majority of the noise is gone, and it was being caused by bad HU rca's. but im still getting a faint ringing tone through the speakers. it doesnt get louder or softer no matter what volume its at, and its still there even if i mute the system. it sounds like that broadcast warning test on tv with all the different color bars, but not as loud. what could it be?