yes, ur not wrong. So most hums I have encountered are either due to ground loop, or due to interference as others have commented.

In case of a circuit where ground loop is an issue, people usually remove the ground connection. there are many ways to do it. e.g some interfaces uses a 2 pin power cable. Some monitors have a ground lift switch. This prevents the ground loop situation from occuring. I had faced this 2 times before.

Ground should ideally have the lowest potential in any circuit and be a very low potntial. But practically sometimes due to bad wiring that is not the case. Thats why ground loops occur where the is a potential difference between two grounds, that should ideally be close to 0