We do not change the shielding, so in your case, there would be no effect. However, hum can have multiple sources from a large unshielded motor, to poor turntable ground to bad motor location, and even incorrectly wired AC plug (as I found out with my Empire 598MKII). Some hum is even mechanical in nature (primarily with belt drives) as 60 hz magnetic fields can find sympathetic routes to induce the mechanical vibrations (which mimic 60hz hum) and can be transmitted thru the belt to the platter.
Wayner