No Sound

For the last few mornings, I've been experiencing a loss of sound from my RNS-E. I leave my car at night(radio last in use), and when I get back in the morning and turn the ignition key, there is no sound from the unit(which starts up in radio mode). There is a feint hiss, which goes away if I mute, and returns when I un-mute. Turning the volume knob makes no difference to the volume of the hiss. I have to swap to mp3, and there is then sound, and back to radio, and the sound works again. Anybody else have this?