How do stop a hum after connecting my mac book to sound system?

Question

hum problem

Answer 1

Try resetting your PRAM.
Resetting the NVRAM/PRAM
Note: This will not delete any data.
The NVRAM/PRAM stores information such as speaker volume, screen resolution, startup disk selection, and recent kernel panic information (if any). You should reset the NVRAM/PRAM if you experience issues relating to these functions.
Shut down the computer by going to Apple () > Shut Down…
Select Shut Down or allow the computer to shut down automatically.
Turn on the computer by pressing the Power Button.
On the built-in keyboard, press and hold the Command (⌘), Option (⌥), P, and R keys before the gray screen appears.
Continue to hold down the keys until the computer restarts and you hear the startup chime for the second time.
Release the keys on the built-in keyboard.
If that doesn't work, try turning down the volume of the external speakers when they're not in use.
I hope I helped.
Austin