Volume goes down on radio randomly

I've had my 2009 Aveo since September, and there's always been some type of bug in the stereo system that's hard to explain without hearing it. I keep it on the radio most of the time, if I keep it in park, no matter how long, the radio stays at its volume. When I start driving, the radio sometimes gets quieter and I have to turn it back up. Sometimes this is a few seconds after I've started driving and sometimes it's minutes. The next time I turn my car on, the radio's a lot louder because of the volume adjustment. This does not happen with CDs or aux, I have been testing this for months and even had audio professionals listen to see if they know anything. I do not have anything special hooked up, it's just whatever came with this car.

I wanted to see if anyone on this forum has experienced this before and figured out what it was. I haven't had time to take it to a stereo install place or have a mechanic look at it, but it's something I'd like to eventually figure out.

I wonder if there is some kind of automatic volume control that simply does not work. I can understand reducing volume on an engine start.