The cables are acting as an antennae and picking up the radio waves. An electrician isn't really the right guy to ask; he doesn't normally deal with any frequency besides 60 Hz and even then they are not trained and are rarely knowlegable about even that when it comes to audio ground (and ground loop) issues. RF (your issue) is right out of his sphere of experience.
The issue you have really has nothing to do with the Mac or your receiver itself; it's an installation or cable length or quality issue and can happen anywhere if care is not taken to eliminate it.
You don't say exactly how you are getting the sound to the receiver; generally if you use a long mini-mini cable and a short mini-RCA adapter, you are using the poorest possible configuration. A short mini-RCA adapter and a long double run of RCA-RCA is better to eliminate hum and radio waves from the signal, usually. Mini cable will be 3-conductor with the shield carrying both channel negative signal, making shield integrity critical. Store-bought mini cable is almost always pretty crappy, no matter what you pay for it.
If you can, try a different cable from the Mac to the receiver's AUX input. Isolators are available but you won't like the price. Especially with longer runs (anything more than 2 Metres) you need quality cable with good shielding. You don't say how long your run is, but if it's 7 meters or longer you will almost certainly not get good performance with cheap cable and runs of 30 meters or longer are headaches more often than not even with special care.
Simply shortening the main cable a small amount (say 5 inches) might eliminate the problem for you; the length of the cable is what makes it a good antennae for that particular radio station. If you can solder, it is a good first fix to try and will cost a couple of bucks at most. If you can't solder, any music shop (you know, where they sell guitars) usually has someone around who can, and it won't cost much to get him to do it. Free, sometimes.
It won't fix the underlying problem (the cable is improperly shielded or poorly constructed) but the "antennae" that is your cable will be the wrong length to pick up the local station giving you problems now. Hopefully it will go away. I'm guessing it's an FM station which is typical. Sometimes you might have to play with the length a bit if you are in an area with saturated FM bands (like, say Toronto).
NEVER bend cables tightly, either in installation or handling and storage. This usually breaks the shield in the middle of the cable, bringing up the issue you're having. It's definitely a bad idea to have cable that is too long for the run; it will contribute to the problem.
Without knowing how or what your installation looks like, it's really impossible to fix in a forum like this; I could kill it if I went to your house but from here all I can do is give you things I would look for.
It's possible the radio waves are being picked up due to poor installation practices as well; if your audio cable runs close to any other cable in your house, it could be picking it up magnetically from that cable, especially house power wiring but also cable and sat coax. I try to keep them 6" apart if I have to run parallel, and even then only as long as necessary to get through some area where it's unavoidable before they get separated further.
When you need to cross another cable during installation, do so at right angles only rather than running parallel. Don't coil cables of any kind, including power cords; use the right length in the first place. Running under rugs or furniture will definitely kill the shield sooner or later.
If it were me I would construct the cable myself, but you might not like that idea or be willing to do it.
If you decide to buy new cable instead, store-bought video cable (like the kind you would use to connect your DVD to the TV) will be well shielded and comes in long runs at a reasonable cost and is available everywhere.
Although it's not perfect for audio it's a good choice for your application and may well be better than what you see in stores for audio (we might call them "far from perfect"), video signals are demanding and the stuff you can buy for video is better than what passes for audio cable in most places. Just buy the 3-cable set and don't use one, and connect the two RCAs to the mini adapter at the Mac side.
Last edited by gordguide; Mar 22nd, 2005 at 10:16 AM.