Radio interference on the line in and line out

Question

OK, this is a complicated one. For the past four months, I had a rather ancient PC. I used this to play music, as it was connected to a Logitech 5.1 computer speaker system, which was attached to the three ‘out’ ports in the back of the PC. I also had an electric piano connected to the PC through the ‘in’ port in the back of the PC. I was able to play the piano and hear the sound through the speaker system.
For Christmas, I received a shiny iMac G5. I quickly set it up with headphones from the line out. Everything worked fine. Then I set it up to use the 5.1 speaker system through the line out. Instantly, I heard a local radio station loud and clear. Furthermore, when I plugged in the electric piano (using the LineIn program), I heard not only the radio station but also a buzz. (When I disconnected the electric piano and used the headphones or built-in iMac speakers instead of the speaker system, there was no problem.)
Now I can recall a couple of times in the past four months where I thought I heard some faint radio station sound coming from the speaker system (then hooked up to my PC). However, it was so faint that I wasn’t even sure it was coming from my own room. In contrast, with the Mac, it is LOUD AND CLEAR. I couldn’t get clearer reception from my clock radio. We do have some major radio towers about a mile away, but they obviously weren’t a real problem with my PC.
The radio/buzz sound coming through the speaker system is unaffected when I increase or decrease the volume using the keyboard controls. However, the radio/buzz sound can be turned up or down through the keyboard controls when I use the headphones instead of the speaker system. The sound is unaffected by adjusting the on-screen volume slider on iTunes.
Granted, when I plug in the 5.1 system, it has three headphone-jack 'ins', so I had to use a connecter putting together two of them into a single 'in' and letting the other one hang loose. The radio volume increases slightly when I touch the loose end. This shouldn't be the source of my problem, however, because I still hear the radio (and buzz) even when I unplug the 5.1 system.
I brought the computer to my housemate’s room and plugged his speaker system in. The radio sound went away. (I wasn’t going to go through the trouble of dragging my electric piano over). But this didn’t do me any good, other than for research purposes, because I’m not planning to park my computer in his room.
I tried using a shorter cord between the electric piano and the Mac. No difference. I tried ensuring that the cord was never vertical at any point. No difference.
I called Apple tech support, sitting on the line for about 45 minutes in all. They could not help me, and could only direct me to the Apple Discussions site.
I read up on the Apple Discussions site, which suggested plugging the Mac into a different outlet. When I tried this, things only got worse. The radio sound was much more staticky. Even worse, it started up as soon as I plugged the Mac in – the Mac did not even have to be turned on. So I went back and tried plugging the Mac directly into the outlet to which the Mac was originally plugged in via a power bar (this time sans power bar). Absolutely no difference. I tried plugging the Mac directly into other outlets in the living room and the kitchen (which also necessitated plugging the speaker system directly into an outlet, rather than through the usual power bar). Absolutely no change. I tried wrapping the cord from the Mac to the speaker system in aluminum foil, as had been suggested on Apple Discussions. There was no change.
I tried using the “Sound” menu in “System Preferences”. I adjusted the output volume. No change. I muted the output volume. No change. I adjusted the input volume. This only increased both the radio and the buzz sound. When I turned the input volume to zero, both sounds continued.
I called my mother, who happens to know a thing or two about sound. No luck. I called my brother-in-law, who works for one of the big three internet companies. No luck.
I'll try to hold off with the editorial comments for now, but suffice it to say that this is my first Mac experience, and any helpful advice would go a long way toward soothing my feelings about Apple right now.
Thanks.

Answer 1

Yes, if I plug the electric piano into the Mac and listen to it through headphones coming out of the Mac, I get the radio sound (as well as a buzzing sound).
However, when I plug the electric piano directly into the speaker stereo system, bypassing the Mac entirely, I get only a small amount of interference/buzz. This makes me wonder if the Mac is, at the very least, dramatically amplifying the interference. (Again, when I formerly used the PC, there was a tiny bit of interference on rare occasions, but it was so faint that 99% of the time I did not even know it existed.)
I've also retried every possible combination in the old PC, just to make sure I wasn't fooling myself before, and at the moment I cannot pick up any radio interference there whatsoever, no matter how hard I try.
iMac G5 Mac OS X (10.4.8)