iMac buzzing noise when screen dims

This message log is quite old ... agreed ... the iMac's also 'suffered' from yellow screens ..... but that is probably like .1% of all sold if not less and then far less is such for the buzzing

There is a very low chance that anything will happen to your iMac but then on the off chance they will help you out and replace it. After Apple taking in my computer ... the 2009 iMac they have replaced it with the 2011 model which is very good

So my machine is working well 🙂 (( as for you buying an iMac .... there was supposed to be a new one before the end of the year ))

Well, after a little searching, I see there are folk who bought iMacs last month (November 2012) and STILL have the whistling. So this would appear to be an ongoing problem.

Whilst this may not affect every iMac, Apple has been selling up to 4 million iMacs per quarter, and a year later customers worldwide are still having the same problem; on these numbers I'd say M12HA3L's estimate of less than 0.1% of machines affected is unrealistically low, and the risk of getting a 'bad one' after all this time is unreasonably high.

Yes, Apple will help you out, up to a point. But many folk have been offered multiple repairs and replacements which still failed to offer a resolution, and that's not a scenario I want to get into at Christmas!

Regards,

Jona

EDIT re my previous post: First paragraph should of course have read "November 2011".

I know this thread is over a year old but it was perfect for me... I had the exact same issue and this answered all of my questions. I sympathize with those who live far from a licensed repair place... I brought mine in to my local store a couple of days ago. The initial "genius" was an older man who admitted to me that he was hard of hearing and kept referring to the sound as a "hum", even though the computer was making the distinct buzzing noise in the store. I was pretty annoyed but convinced him to keep it overnight, and he called the next day saying one of his colleagues had heard it. **Apparently their technical manual states that the problem is most often related to the backlight board**, so they started by replacing that first, but found that the buzzing was still there. Then they went directly to the PSU and found that it was the direct source of the sound, so they replaced that also. I just brought it home and fired it up a few mins ago and so far no issues.

Thanks for all your work on this!

Although this is an old post I would like to add in an additional cause of the noise as I too had this issue for a while and would just turn the brightness down a bit. I just had my Power Supply replaced and subsequently the brightness issue was also resolved. Apple Support looked into the noise and determined it was gone once the power supply was replaced.

This might seem a strange answer, but it has fixed this fault on my 27" two year old iMac (at least for a few days now, will be monitoring).

I had this very fault suddenly develop last week. It was consistent, the noise increased with screen brightness, but was also present with the screen asleep.

I looked at every forum on the subject, became almost convinced my fans weren't controlling properly as well.

I decided it was the switched mode power supply, discussed repair options with my local Apple Store.

Checked the household mains voltage - I'm in the UK, it was slightly low at 236v AC, but thats acceptable.

Then ....

We have a garden full of water features and LED lighting.

I just decided to check all our power junction boxes that supply power to various pumps and lighting.

I found one of the boxes full of water, with the Live, Neutral and Earth, and the 12v DC cables all under water and shorted together. The terminals were all corroded and burnt.

Yet, I had no faults present in the garden, everything was running normally. And the RCD breaker had not tripped.

I repaired the box, fitted new terminals and rewired the box.

And since I did that, my Mac has cured itself.

I've had it running for hours to get hot, no noise, gone completely.

And the mains voltage now measures 242v AC (should be a nominal 240).

The RCD did not trip because, with all the cables shorted in water, there would no out of balance condition to initiate a trip.

So, I can only assume that arcing or the short was causing some harmonics on our mains supply that was been picked up by the coils in the Mac power supply.

The 12v DC supply in the junction box was also supplied by a switched mode supply. I dont understand why something didnt blow in the garden, but it didnt.

I'm monitoring the Mac as the solution just seems unrelated.

Oh, and I noticed our Intruder Alarm panel was humming loudly and now it isn't.

So, I would suggest anyone with this issue takes note of any unusual humming or buzzing in any other electrical item within the house, or outside. It may indicate you have an issue elsewhere.

Even any loose mains plugs in equiment could be causing a slight arcing on the contacts, causing noise on the mains that may be getting picked up.

Coming from an interest in high-end analogue Hi-Fi, amplifiers always performed better with a dedicated, clean, mains supply, usually from a 40 amp rated cable fed direct from your electricity meter to supply your Hi-Fi.

The amplifiers would exhibit less noise and a cleaner sound with more detail. And the transformers in the power supply were a lot quieter too.

And what do we have today - ethernet tranducers plugged into household sockets, superimposing high frequency signals onto the household wiring.

They would cause havoc with top-end amplifiers, I'm sure.

I hope this is the solution, saves me an expensive repair.

I've never had any problems with the six Macs I've owned in the last 12 years.