I got a new 30" Apple monitor today, installed it and it looks lovely. Will make production easier as I’ll be able to see more of the arrangement view now.
However, somehow it’s created an intermittent ground loop so I get an occasional low whine coming from my studio speakers which is VERY noticeable when I plug headphones in. Dammit
Gonna have to strip away each part of the set up now to try to track it down. Didn’t happen with my older smaller Apple cinema display.
Anyone got any good suggestions - not involving getting a PC! 
Hmm it’s not the speakers as I get the same whine if I plug my headphones directly into the Macbook.
Must be the new cinema display. Maybe it’s the magsafe powerfeed coming into the macbook from the display itself. Previously the macbook was powered by it’s own supply.
ah well, that was simple enough in the end. External hard drive and new display both plugged into same surge protected power block. Unplug external drive plug in elsewhere, sorted.
I have the same issue… **** knows what it is or where to start… i’ve gotta put up with it, i dont know enough to go trackin it down. care to fill me in?? 
Get a PC
Hey Wayne
I’m no real tech head either. basically though just unplug everything and I mean everything - including from the wall, as it’s usually a grounding issue.
Then, one by one add your hardware back into the set-up loop.Making sure you take some time between each new bit of kit being plugged in and switched on. Listen for the whine starting again. As soon as you hear it, it’s probably the last thing you plugged in interfering with another piece of kit.
Try moving the cables around, try plugging that unit into another power socket in a different part of the room (if you can) and so on.
For me it was a case of the new display ‘clashing’ with my external back up drive. They were both plugged in to the same 4-way surge protected power block. I unplugged the drive from the block and plugged it into a different one. That solved it.
Oh, and Rob… bugger off