DKA790GX Platinum + Microphone noise {fixed - rework}

Question

Hi guys,
I had troubles with the above combination, high pitch noise in the Microphone input when Mic is connected.
So I went with my board to MSI EU to test it there, no noise.
I took it home, noise again.
Changed the powersupply to Enermax 365 as MSI used an OCZ 850, no noise.....
Whoooot?
So I contacted OCZ (PC Power & Cooling = OCZ) and after a few emails they decided to RMA the powersupply.
After a few weeks I got a brand new one, connected it, no noise YEAH!
Within 2 weeks the trouble started again, noise, replaced the powersupply to Enermax 365, noise gone.
Ok, now I'm getting nuts! So I called MSI EU and told them I was on my way with my Silencer 750.
MSI had a board prepared when I got there, different memory, different 90nm CPU...and Windows XP (I changed to Vista as the first test of my board at MSI was done with Vista)....no noise!
I looked closed, and they forgot to connect speakers
So I hooked up a couple of noisy boxes and Whooot!! noise on the mic again.
MSI was as surprised as I was and they thought OCZ had simply returned my PSU, so they took out another Silencer 750....bingo Also noise!
So we tested mine and their PSU on several other boards, no noise.
The noise is about 8KHz and is only there when a mic is connected (and mic is un-muted so you can hear it via the speakers).
For some reason this combination does this, there are no other problems, no crashes nothing.
MSI EU is forwarding this problem to MSI Taiwan so they can reproduce the problem.
It only happens with this combination, so in case you have it, best replace the powersupply to some other type and it's fixed.
We do not know what the reason is, but it happens, BIOS doesn't fix it as far as we can see.

Answer 1

The powersupply MSI had will be send to Taiwan together with the tested board.
I believe the problem is a combination of factors.
Anyway, MSI probably has to alter the motherboard so it doesn't happen, as other boards don't have the problem.
Mike we tested my board and another board MSI EU had, both doing the same, with my PSU and MSI's Silencer 750...same thing happened.
Both boards where tested with different PSU's non PCPC and the problem wasn't there.
I do believe the problem is the board not the powersupply, as the board should filter spikes, but doesn't.
And the setup on the board was different:
Mine <-> MSI
4200+ 90nm - 5000+ 90nm
Kingston DDR667 2GB - Something with heatspreaders
Samsung 160GB Sata - WD something Sata
Stock cooler - Some fancy cooling, not fastened but cools.
Also tested, onboard video and Ati 3870, 4850, nVidia 2xx
And other soundcards, also noise.
And different OS, Vista and XP, random order.
Too much is different to believe it's a small problem.
Mike we are sure that the combination causes this, very sure.
It's my believe the board starts oscillating on the buses due to the resistance of the powersupply.
Why exactly, don't know, that's for MSI and OCZ to find out, but it does.