Audio distortion on 24" iMac

Hi all, I've got a real puzzler of a problem and I'm hoping you can help!

I have a 24" Intel iMac, with the latest version of Snow Leopard. I bought it last July and have AppleCare. For as long as I can remember I have this problem with it:

* Every 30 seconds, almost to the second, the audio from the built in speakers distorts and gets slightly louder. This lasts about half a second then goes back to normal
* It only manifests through the internal speakers, it's fine through headphones
* Rebooting makes it go away temporarily
* I've reset PRAM and SMC to no avail
* I've tried turning Time Machine off to no avail
* I've taken it into my local Apple Service Centre, they couldn't replicate the problem, it only seems to happen in my house (every room I've tried it in is the same)
* I bought an expensive power filter/conditioner, this hasn't helped at all

I'm at my wits end with it! Please can one of you clever folk give me some suggestions?

Thanks!

Sounds like some sort of interference that may be causing it. What leads me to believe it may be interference is the regularity of it occurring - every 30 seconds or so. Power filters and line conditioners are generally a waste of money when dealing with interference, especially if it's some sort of RF signal which is causing it. You're just going to have to be a better detective and try to locate where it's coming from.

And... please don't commit seppuku over it. (where did you come up with that handle?)

Regards.

Hi, thanks for your reply!

OK what are sources of RF - radio/TV broadcasts? Anything else?

Oh and I've used this handle online for years. Comes from one of my 'musical' projects: seppuku on MySpace Music - Free Streaming MP3s, Pictures & Music Downloads

OK what are sources of RF - radio/TV broadcasts? Anything else?

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Any amateur radio operators nearby? Live near an airport? Any electronic security devices installed in your home? A medical office in the neighborhood?

Since whatever it is that's causing the problem has no effect on the audio system itself (headphones are OK), it has to be something that's modulating and coming only through the speakers. Speakers are magnetic devices and can pick up all sorts of interfering noise.

Out of curiosity, have you tried moving your iMac to another location away from your home - like at your office? I know it's not easy to pick it up and carry it to work, but might be an interesting experiment to see if it's really interference around your home.

Regards.

Nope, don't live near an airport or hospital or anything like that, and I'd be surprised if there's any ham radio fanatics on my street as it's mainly old folk.

One thing that does confuse me a bit is that the problem goes away if I reboot, then gradually comes back. I sleep the mac all day and it seems the longer it goes without a reboot, the louder/more annoying the distortion gets.

I've not tried taking it to work but I was considering it.

Oh also, I have a 13" macbook, which I run in the same room, from the same power socket, with the exact same software on and the same hardware plugged in and it has never had this problem.

I've not tried taking it to work but I was considering it.

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If you have the time and energy to do it, it would give us a better idea of how to track down the cause. If the machine is OK at the office, then we know the problem is local to your home. If it does the same thing at the office, it would have to be a hardware problem rather than interference. Could be something as simple as the speakers (one or more) being intermittent.

Regards.