Occasionally, I'll get this problem too when the engine is running. Most of the time, it is perfectly quiet.
If I connected my Alpine CD up, I get no whine at any volume.

I traced the problem down to the pin in the connector on the sled not being seated properly, creating a ground loop condition. I'm not very knowledgable about electronics so this is only my speculation.

If you removed the sled and check the pins, hopefully, this will fix your problem.

Be careful though because I actually severed the wire from the connector when I was playing with it and had to re-crimp the thing.

I've been meaning to order a new sled just to avoid this in the future given my breaking the original factor connection.