I recently got a 2006 Mercedes e350, passed it's Mercedes certified check up a week ago so it's electrical should all be in working order.
The last couple days I noticed the volume of the sound system randomly rises/falls in odd cycles. It fades up and down, sometimes almost inaudible, other times way too loud. Adjusting the volume when it drops/rises just means the next "cycle" will be even more inaudible/ear splitting. It's happened consistently for the last two days; best I can recall it never happened before then.
I'm not accidentally hitting the volume buttons on the wheel and the volume knob on the main console stays firm. Both the buttons and the knob work fine, and I've reset the sound system to default settings a few times. There's no other settings (that I can tell or saw in the manual) that would affect my sound.
The problem has been with playing CDs, the Radio and the Aux input jack, so it's all sound.
What would cause this and how can I fix it?