As I work in the Pro-Audio industry, I am quite familiar with the term and causes of 'Ground Loops' (and what
this piece can resolve) so I will just address that first.. Then I will try to toss out a couple of things to check if you are able to..
A Ground Loop is essentially created when 2 or more electrically powered devices are connected that enable more than one path to an A.C. Power Ground source ~ ~ ie. If you had an issue with your iMac and a set of Powered Speakers, as opposed to headphones, then that device in the Link above, which likely has an isolation transformer inside it, might 'disconnect' the actual devices physically by passing the Audio Signal through the
Transformer Coils.. Technically, 'the loop is broken' by having the Passive Coil remove the linking of the devices, the Audio passes through by proximity of the Coils to the Iron Core..
Now your RF (Radio Frequency) Interference issue could be almost related in another way.. Bad Cables (especially if the are very long) and maybe even a Radio Station or Transmitter Device nearby could easily be the cause of you and others hearing this unwanted music.. As 'seanm9' noted above it may well be a common piece such as a Metal Desk, or even Metal Studs in the Wall, that are all combining to 'create an antenna' that is unfortunately sympathetic to the Frequency of some Radio being transmitted somewhere nearby!!
All electronic equipment sold these days has a disclaimer that says what THEY put out and can be affected by as far as electronic interference (usually meaning they can be used near each other without creating issues) but most do not state how well they SHIELD from it though.. Chances are very good that if there are several computers affected similarly that it is a RF Field nearby, and that you will not easily fix the problem, but you never know.. You might try listening on the headphones while rotating the computer on the desk (if it is a true stray RF source I would expect that orientation of the device may affect the sound level, unless a REALLY strong signal)
The World is currently awash in RF creating devices, which is why Portable Pro Audio Systems (such as you would see at any large concert) are always susceptible to so many 'buzz and noise issues' when they are Set Up and Torn Down in different venues days after day.. and why Audio Techs spend literally HOURS tracking down such issues these days, given the massive amount of Wireless Microphones, Instruments, and In Ear Monitor systems that are used in most concerts!!
Good luck, I would first suggest trying to see what is a common component among ALL computers that have this issue, and if there are ANY that Do Not have the issue, what is different about them (same or different headphones, position in the room, etc) ~ If you were near to me, there are tools we have that will show RF Signals and would help to track it by it's Frequency ** If you listen and could learn the stations 'call number' or name you might find out more about The Source.. as stated earlier, you just might have a Transmitter/Repeater near you (if you are in a tall building downtown in a large city, there might be one on your roof) and maybe you all are just unlucky enough to be bombarded with RF that will NEVER go away!!
