Take a look here if you have an Intel iMac

I actually have an AC power supply for my Zaurus PDA that does this. If the PDA is plugged in and on, no whine; if the PDA is plugged in but shut off (and its battery is fully charged), it's a real high-pitched whine that, once you notice it's there, will drive you nuts.

Are you in Europe? I ask because I am, and the high-pitched whine goes away when I'm in the US on 120v current.

In other words, it could be a safe but annoying issue with the auto-switching power supply.

If you don't have the recommended voltage going to your mac it could be causing issues. Try buying a UPS backup, you can get a belkin 550va on buy.com for $50 USD.

I've noticed it in the early morning and only if I put my head very close to the iMac, and agree with the assessment that it's probably the switching power supply. I'm quite sensitive to high pitched whines, I can tell when a CRT/TV is on but displaying a blank picture, so it doesn't seem too bad that I don't dwell on it with the Intel iMac.

B

iMac

I have a 20" iMac and haven't noticed any noise whatsoever.

I can barely hear the Hard Drive, but that CD is a different story!

Mike...

I agree that it's a good idea to use a high quality power conditioner with any
sensitive electronics.

When I check voltage before installing an electric meter, the readings vary
quite a bit.

The readings can be off by as many as 3-8 volts depending on the cable and the distance to the transformer.

I haven't noticed any strange noises with my intel iMac, though I occasionally get some backlight flicker.

Apparently this is "normal"

Guys, my 20" imac does this and it drives me bonkers cos I can hear it at night.

I took into the apple shop and the technician said this is normal and that all electrical devices do this. (Which is utter rubbish-my last computer was dead silent whenever it was off.)

I was annoyed that I had to lug the thing back and be told this. He said that power was still being used by the device when it's turned off or asleep and that what you can hear. Then why don't I hear it when it's on and awake?

mac000 said:

I am wondering from other users if your Intel iMac has a high pitch noise when it is in "sleep" or "shut down"....

On the back left side of my imac near the vent, there is a rather high pitch noise that sounds whenever teh computer is shut off or in sleep. I am debating whether to take it in to be checked out. Its not a huge deal, but if its a start to some hardware failure I would like to know.

Anyone else hear this noise?

Click to expand...

I hear it. It is nnoying(my imac is in my bedroom.

I don't hear anything. Could be my 39-year-old ears, though.

emmakins said:

Guys, my 20" imac does this and it drives me bonkers cos I can hear it at night.

I took into the apple shop and the technician said this is normal and that all electrical devices do this. (Which is utter rubbish-my last computer was dead silent whenever it was off.)

I was annoyed that I had to lug the thing back and be told this. He said that power was still being used by the device when it's turned off or asleep and that what you can hear. Then why don't I hear it when it's on and awake?

Click to expand...

It is because of the fans. You know how projectors cool off when you're finished using it? It is like that. It is a CPU fan, but not the main one. It is a small fan. There is also one by the Hard Disk.

My 20" iMac is quiet no matter what so maybe I'm lucky or it is working just fine. The sound (fans) only goes up if I'm doing a heavy load in Flash but not enough to bother me and really, iTunes takes care of any sound that I would complain about when doing so.