First I had separate grounds for CarPC and amp and I got processor noise and alternator whine.
Then I ran a ground wire from the amp to the CarPC and things got better, but not perfect.
Then I ran a fat ground wire from the battery to the amp and left the ground wire from the amp to the CarPC, now it's perfect.
Before I used the battery minus for ground, I used a seat bolt, which is a bad idea.
Wisdom: A bad ground can mess up things, even if everything is hooked up to it.
You should at least for testing purposes hook every damn thing directly to the battery minus. If that cures the ground loop, take it from there.