obviously, this shouldn't be an inherent problem.
The first thing I'd do is to move the imac to a few feet from the imac and try a much shorter cable. This will isolate if the problem is the cable and how it's running or in the IMAC itself. Note: you will most likely be plugged into the wall on a different circuit, or at least the same circuit as your sound equipment...which also eliminates the power in your home.
If the buzz goes away...run an extension cord to the original plug you were using with the IMAC (to see if it's the home electrical system you have to deal with or it's internal to the IMAC.
Then you know what problem to fix. If it's noise on the power line, get a line filter; if it's the IMAC, good luck discussing with Apple. If it's the long cable then you may need to run a balanced cable or reroute it.
Good luck...
