My Kenwood DVR-5070 makes a loud buzzing/ringing/feedback noise when it is powered up, and the noise continues until a dvd or tape is played. Think smoke alarm. Once playback starts, the noise level decreases, but you can still hear it in the background. This is a combo receiver/ dvd player. The noise stops when the speakers are disconnected. I disconnected one speaker at a time, including the subwoofer, with no change. I disconnected the vcr from the receiver, one cable at a time, no change. I also disconnected the am and fm antennas, no change. Any ideas?