Found this great article written by Rane Technical Staff in 1985:

• Cause and prevention of ground loops
• Interfacing balanced and unbalanced
• Proper pin connections and wiring
• Chassis ground vs. signal ground
• Ground lift switches

The article can be found here

A PDF version can be downloaded here

This article explains the cause of audio hum and possible way to prevent/ solve the problem. a must read for any audio people.