Noise isolators are about 5 to 29 bucks on amazon
The radio in my offroad vehicle does the hum you describe due to a loose wire at the amp.
It's behind my glove compartment so I just slam the door and it quits for a while.
Mine does it at the same volume even when in map or some other mode than radio. I suspect it's the common ground for the speakers getting some power from somewhere, a knicked wire maybe.
Loose wire jiggled and the stuff in the link installed should fix you up.
It bothers the hell out of me, but I don't want to delve into the wire nest bad enough.
I think any stereo install place would have it fixed in fifteen minutes.
As far as humming goes:
Show tunes is the answer... Show... Tunes....