iMac buzzing noise

My late 2009 27" iMac developed the same problem, a noise that sounded like a worn or dirty fan that increased with screen brightness. Using Mac Fan Control I was able to eliminate the fans as the source of noise. Instead I concluded it emanated from the power supply. Resetting SMC and PRAM had no effect

I usually put my Mac to sleep, but shutting it down and unplugging the power cord overnight has taken care of the problem. I now do this instead of just sleeping the Mac a couple of times a week.

Noises are difficult to diagnose and fix without seeing or hearing the machine. You can do some basic resets including restarting in Safe Mode to clear some caches and also resetting the SMC and PRAM. If your machine is covered by AppleCare then call them otherwise book an appointment at your local Apple Store and take it in to be looked at.

SMC RESET

• Shut down the computer.
• Unplug the computer's power cord and all peripherals.
• Press and hold the power button for 5 seconds.
• Release the power button.
• Attach the computers power cable.
• Press the power button to turn on the computer.

PRAM RESET

• Shut down the computer.
• Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
• Turn on the computer.
• Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
• Hold the keys down until the computer restarts and you hear the startup sound for the second time.
• Release the keys.

I too had this problem but I found a solution. My noise would even change as I moved the curser around.

I did a lot of searching around but ended up finding the solution by accident. The problem, it turns out, didn't have anything to do with my screen. It was a fluorescent lamp plugged in to the same circuit as my computer. It was actually on the other side of the room but it creates interference in the AC power that conflicts with the power supply in the computer. Somewhat similar to what makes some car stereos buzz when driving.

I have played around with it since and changed my desk lamp to a CFL bulb and even though it was plugged in directly beside my computer it didn't make the noise. It turns out just some fixtures do it, and from what I can tell, ones with a dimmer switch are the worst. So, I would check to see if you have your computer plugged into an outlet that is on the same circuit as a light. Even an overhead light such as above a dining room table. It sounds weird, but it depends on how your house is wired because that light doesn't even have to be in the same room.

Surge protectors may help but unfortunately you may have to go to a more expensive current modulating power bar like the ones made for high end stereo equipment.

For me, it was just a matter of moving a lamp. I hope this helps someone out there.

gurton, when you discovered it was the fluorescent light, was this a light you had just installed? I ask because the noise just started on my iMac but I haven't added any new electrical to my house. So for you, did you just install a new lamp and then noticed it started happening, or did you have the lamp all along, and the iMac started with nothing new like me?

AWESOME!!!!! I had to run an extension cord to my imac, cause my room light was on a dimmer and that buzzing sound was DRIVING ME CRAZY.

Getting my computer off the circuit with all the stuff and dimmer light switches WORKED FOR ME!!! thank you GURTON!!!!

NICE FIND!!!!