[Veltliner]

My 24" white iMac has got a hum.

It is of rather low frequency, a bit like a fog horn, and can be heard across the room if it is quiet. I work a lot late at night, when it is quiet, and then this can get really annoying.

It has had this from day one for the first two days, then it stopped, and after three weeks it started again.

Starts a few minutes after starting the iMac. It does not sound like the swishing of fans.

Noticeable: when you put the display alone to sleep, the sound stops immediately. (and, of course, when you put the whole computer asleep).

Has anybody noticed anything like this?

[adamfishercox]

Sounds like a display issue if it stops when the display is off. See if you can get it to go away any other way.

Try:
Logging Out
Screen Saver
Unplugging any peripherals

[MacNNUK]

Sounds a bit like the old CRT mains frequency hum when over bright caption was on screen.

Does it happen with any particular application / website.

Does it reduce when partial brightness drop kicks in ?

Possibly mains transformer probs when driving screen.

Or switched mode power supply ?

Vertical frequency of TFT screen in region of 25-120 hz , could it be in this frequency band ? (Not sure what iMac refresh hz is) 67.7hz ?

This may help..... Apple - Support - Search Results

or this.... Apple - Support - Search Results

You could always record it using iSight, then comparing frequency in an audio editing programme, or even with a tone generator, to assist in locating the source.

[Veltliner]

I have the impression that it has to do with the screen.

But not with the heat. If it would be a heat problem (a damaged fan), then the fans would spin up first when the screen goes dark.

So possibly something in the power unit providing the screen with energy, and it has to warm a bit to start humming. It really sounds a little bit like a transformer.

[Veltliner]

This is really weird.

I had my iMac at the repair shop for a day. Hardware test: no problem. Temperatures everywhere normal. They even opened it up to see if some piece of paper or plastic wrapping was left inside from building: nothing found. The fan sitting behind the center of the screen works well.

I disconnected everything from the computer - no avail.

Now I am sitting in front of my iMac, and it hums. It is really annoying.

It sounds like a fan is spinning all the time (I can hear it clearly on the back side of the iMac). But fans usually don't have a low frequency, whining hum.

As I know iMacs are very quiet (my old G5 2.1 ghz never made the tiniest noise) I know something's not as it should be.

The weirdest thing is: as soon as the screen turns off, the sound stops. When you put the computer asleep, the sound stops immediately (fans usually spin up a bit, and only then stop).

[MacNNUK]

You could try using a tracer...

Basically, a pencil, with blunt ends, one end against ear, other end against spots on iMac, till you trace the source of the hum.

I obviously don't recommend you put the pencil end into your ear.

You could glue? a larger end to the ear end, if that makes sense.

Or one of those pencil rubbers, or even a pencil with rubber fixed on, best option yet.

Try a stethoscope, if you can borrow one.






Does the hum disappear when you select sleep, as the screen goes black, before the whirr noise ?

Does it appear whilst the screen is still black, when awakening from sleep?

Does the hum change if you place the iMac face down ?

Or if you flex or squeeze the casing.

What happens when the screen dims, prior to energy saving switch off.

Does the hum go when energy saving kicks in, (ie screen off) or only when you sleep the iMac ?

Do you have energy saving enabled.

What desktop background...etc

Could it be something else, even the speakers, affected by the screen brightness.

Hearing aid for example ?

Fillings in teeth ! It has been known.

Mains powered radio / clock radio nearby, picking up HT from screen.

Printer ?

Modem etc

Desklamp


If you turn away, back to iMac, same distance does it sound as loud ?


Have you tried reducing brightness in setup, does this affect it.

Can anyone else you know hear it?

Have you tried it in a different room / house.

It could be "beating" whereby two slightly different frequencies interfere with each other , giving a sum and difference beat.

ie 10,000hz & 10,060hz audio would give 60hz, and 20,060 hz as well.

Thinking 15,625 & whatever US TV horiz freq is, or mains beating with monitor hz refresh rate.


And finally...could the tech hear the hum before they opened up the iMac.

I once had a terrible time trying to show a TV repair man that there was a purity error on an old TV set.

He just couldn't see it, and it was really obvious to me.

Have fun.

If all else fails, turn up iTunes !!

[Veltliner]

When the screen dims (just before it goes asleep), the hum gets louder.

When the screen goes asleep, the hum immediately stops.

Screen brightness has no influence. I also unplugged everything. No influence. It can be heard from 10 feet away at night. During the day, when there is noise coming in through the window, you can't hear it.

But I work at night, hours on end, and the hum makes the computer practically unusable.

[Veltliner]

When the iMac goes asleep (the whole computer, not only the screen), there is an Rrrrrrr-noise, same when it wakes up, but I guess that's the normal hard drive noise.

[MacNNUK]

Originally Posted by Veltliner 

When the iMac goes asleep (the whole computer, not only the screen), there is an Rrrrrrr-noise, same when it wakes up, but I guess that's the normal hard drive noise.

Sleep ...quiet Rrrrrr

Wake up....louder Rrrrrr

[Veltliner]

Originally Posted by MacNNUK 

Sleep ...quiet Rrrrrr

Wake up....louder Rrrrrr

I have to verify that.

But I found one other thing.

When I dim the screen brightness using the f14 key, the hum gets louder.

So:

Screen dims before sleep - hum louder.

Dim screen with f14 key - hum louder. The f14 key is almost like a volume button for the hum.

Also: It takes about 50 minutes to get to maximum hum intensity.