To anyone having this hum issue, try this just out of curiosity (if you haven’t already)…
On Windows 7 based systems (no idea about Mac) go to your control panel, then go to ‘power options’, then ‘edit plan settings’, then click ‘change advanced power settings’.
A dialogue box will open in a new window called ‘power options’, open the branch ‘USB settings’, then the branch ‘USB selective suspend setting’.
Here it should have two options, one is ‘on battery’ and the other is ‘plugged in’, set both (or just ‘plugged in’) to ‘disabled’.
Also, go to the branch PCI Express, open it, then open ‘link state power management’ and set it to disabled as well on ‘plugged in’.
You can also set ‘turn off hard disk’ to ‘never’ by entering the value of ‘0’.