Buzzing/humming noise from speakers when using integrated sound

[D4L41L4M4]

So yesterday I've got myself a new PSU, after cleaning all the dust, reapplying the thermal paste on the CPU, and installing the new power supply, I boot up windows, and as soon as I turn the speakers on, I can clearly hear a buzzing/humming/coil-whiny noise coming through them. And like that the lengthy troubleshooting has begun.
One thing to note - I'm using a Hyper 212X CPU heatsink, and when I was removing it from the CPU, one of the nuts that attaches the back-plate unscrewed, so I had to pullout the mobo from the case in order to retrieve it.
So here's what I've done:
1. Swapped back to the old Power Supply. Still buzzing
2. Reinstalled the audio drivers. The problem is still present.
3. Disconnected mouse, keyboard, mic and ethernet cable. Didn't help
4. Tried to plug the speakers as rear, or side. Same noise
5. Instead of speakers, plugged in the headphones. Couldn't hear the noise, but the headphones are quieter, so maybe that's why.
6. Moved the treble/bass/volume knobs around, I've been using the speakers with all of the knobs set to 50. I've noticed that when the treble is set to 0, or volume to ~20 - 25, you almost can't hear the noise.
7. Plugged the speakers into the front of the case. The noise is gone, or I can't hear it.
8. Decided to test, if graphically intensive games affect the noise in any way. Ran Far Cry 3 - electrical whiny noise when looking around.
9. Changed the GPU to my old 5670. Buzzing still present.
10. Tried changing the power outlets. Didn't help.
11. Plugged in my old Sound Blaster audio card (SB0230). The noise is gone.
Also a couple of things to note - the noise starts only when in windows, everything is fine when in BIOS, or the PC is turned off. Heavy HDD activity seems to affect the noise by making it quieter, and moving the mouse around when dragging windows or audio slider changes the type of noise that is coming through.
So what could be the cause of this noise? Is it the motherboard itself? If so then, what repercussions there may be? Is there a way to fix it, so I can keep using the integrated audio?

My specs:
Motherboard - p8z68-v pro/gen3
CPU - 2500k
GPU - Sapphire 7970 Vapor-x
RAM - 8gb Kingston Hyperx
HDD - 1TB SAMSUNG Spinpoint F3 7200RPM, 500GB Western Digital WD Blue 7200RPM.
PSU - Seasonic x650. (Previous PSU - Corsair CX600)

 

[Slobodan-888]

See is you can find some big coil with ferrite bead (from some old PSU, for example), take off the wire windings from it, take the ferrite bead and loop the speaker cable through it (with as many turns as you can). That should decrease or even completely remove noise.

http://www.inductancecoil.com/photo/pl9831-odm_circuit_breaker_automatic_transformer_air_coil_inductor_with_ferrite_bead.jpg

 

[D4L41L4M4]

See is you can find some big coil with ferrite bead (from some old PSU, for example), take off the wire windings from it, take the ferrite bead and loop the speaker cable through it (with as many turns as you can). That should decrease or even completely remove noise.

http://www.inductancecoil.com/photo/pl9831-odm_circuit_breaker_automatic_transformer_air_coil_inductor_with_ferrite_bead.jpg

I think that I should have an old, useless power supply lying somewhere, will try it out, and report on the results. Though, I'm not sure it'll help, since the sound was perfectly fine before the new PSU installation.

 

[Slobodan-888]

Bad PSU will produce more noise on its output and that can be picked up by the audio circuit and the audio cable. This solution will reduce the noise, but by how much I don't know.

Wrap ti something like this:

The cable end that goes to the speakers, if you can.

 

[D4L41L4M4]

Well it seems that I fixed the issue by myself, so if someone stumbles on this thread via google search, here's the solution:
Since I couldn't find the useless PSU at my place, I decided to wait until Monday and ask around at work, a couple of my coworkers usually have some old pc parts laying around...
In the meantime I decided to fiddle around some more, one thing led to another, and finally I decided to loosen the screws a bit on both the motherboard and CPU heatsink, and what do you know - it fixed the buzzing/humming noise!