Power requirements are usually set by what kind of cards you are using in your computer, with the main one being GPU's these days, you also get better quality components in the higher wattage power supplies, and better surge protection etc, which is why I run 1200W instead of a 850W or lower that I could use.
 
Some information on GPU power requirements
http://www.realhardtechx....x_archivos/Page362.htm

In addition to the components in my sig, the computer also has an nvidia gtx970, 2 ssds, and 3 sata drives.  So I like to aim high when it comes to power.  Sounds like 850 is not considered high anymore!  A 1200 would be mega expensive here in NZ though!

I agree.  850 watt is plenty for that system.
I have more hardware in mine and only have a 650 watt and it is not overloaded.

I've been coasting along for a month and a half now, not worrying too much about the squeaking until today, when I turned on my speakers for the first time in ages.  Then I heard the squeaking amplified over the speakers.  Yuck.
 
The speakers are connected to my M-Audio Delta-44 PCI card.  So are my headphones; if I turn them up REALLY loud, I can hear the squeaking on the headphones as well (but I never turn them up that loud normally, so the squeaking is not an issue when using the headphones).  Anyway, this means the Delta card is somehow picking up the squeaking sound and amplifying it.
 
My other sound card (Xonar) is not picking up the noise.  This is strange, since the Xonar is cheaper, and is positioned even closer to the power supply than the Delta card.  Hmm...
 
I suppose I could try moving the Delta card to a different PCI slot, but I am worried about creating other IRQ conflict (or are those a thing of the past?)  Also, my case is pretty crowded, so I don't know if there is a free PCI slot in there.  I'm not very confident that moving it to a different slot will help much, since it's picking up the noise when it's 2 slots away from the PSU (and the Xonar is not picking up the noise, despite being adjacent to the PSU).  This, plus the fact that I don't enjoy moving hardware around, makes me reluctant to experiment by moving the Delta card to a different PCI slot.
 
Are M-Audio Delta cards known to amplify computer noises more than the average sound card (e.g., the Xonar)?  Or is there a chance that certain PCI slots are more susceptible to picking up noise than others, regardless of proximity to the PSU?  What else could explain why the Delta is picking up the noise by the Xonar is not?
 
Obviously, if I can eliminate the noise, the Delta-44 would have nothing to amplify (for now)... but if the noise turns out to be something like the motherboard or the graphics card (expensive!) rather than the PSU, then I am unlikely to ever fix it.
 
I'm a bit frustrated at this point, as this noise is discouraging me from using the speakers, which I would like to use for mixing my songs (instead of headphones). 

I did some more investigating and found some other clues:
 
1) When I drag the mouse (hold left click while moving the mouse), I get the squeaking sound in my speakers (and also, faintly, from the back of the case).  Just moving the mouse doesn't cause any noise, and just left clicking doesn't do it, but the dragging does.  I can even have an empty file, and move the mouse (which doesn't affect the display) and it still squeaks.  It squeaks more if there is text in the document (something to highlight).
 
2) Dragging the mouse causes noise if I am selecting text in a document (or files in a folder), but not if I am moving a window around.
 
3) It doesn't matter how loud the monitoring output on the Delta 44 is; the squeak is always the same volume.  If I adjust the volume knobs on the speakers themselves, though, the sound gets louder and softer.
 
4) It doesn't matter if the mouse is plugged into a PS/2 port (via an adaptor) or a USB port; the noise is the same.
 
5) I recorded a few seconds of output from my Delta while triggering the noise with the mouse.  I then normalized it so that you can hear the sound well.  Here is the audio:
 
http://tripalot.com/issue...160927-delta-noise.wav
 
As you can hear, the left channel has a loud hum.  In the right channel you can hear the squeaking sound.  To me, it sounds like one of those old dot matrix printers.
 
So, the Delta is picking up the sound (and that hum), and the speakers are picking up the sound, but the volume of the sound isn't affected by the monitoring level sent from the Delta to the speakers.  Very strange!!!
 
Do these clues help identify the possible cause of the noise?  And, ideally, a fix?
 
 

I've been doing some more tests, and I'm pretty sure the problem is coil whine in the power supply. 
 
The main "clue" is that when I change the power mode to "power saving", the buzzing mostly goes away.
 
So, I'm now back to trying to decide which PSU to get as a replacement for the Corsair DX850.
 
Since the Corsairs are notorious for coil whine, is there a particular brand that isn't (without costing too much)?

I received a new sound card, a Focusrite Scarlett 2i4.  It uses USB (while the Delta is PCI).

I plugged my speakers into it, and am getting the same computer noises.  And, as with the Delta, it does not matter how high the Scarlett's monitor output knob is set. 
 
So, as long as the speakers are plugged into either the Delta or the Scarlett, they pick up the computer noise. 
 
This is very frustrating!

I'd swap out the power-supply.
If it were fan related, it wouldn't be "transmitted" thru the audio interface outputs.
If your power-supply is in the lower back section of the case, that's almost surely the culprit.
 
Ground issues are also common sources of noise.
 
I've seen poor quality USB cables introduce "digital hash" type noise.
 

Yeah, I think the power supply is the source of the noise.  It doesn't bother me too much, except that the noise appears on the speakers too, which is MUCH louder than the noise out of the back of the case.  That's why I was hoping that by switching from the Delta PCI card to the USB Focusrite interface I'd be able to avoid the speakers picking up the noise.  But that's not the case!
 
Could balanced cables help in a situation like this?

I tried a different mouse, and connecting the mouse to different USB ports.  None of that made any difference.
 
Also, when the computer first starts up, it always lets off long SQUEEEEEALS, and this time I had my speakers on and hear the squealing really loud in them too. So, every time the back of the case makes noise, the speakers amplify it.