Realtek AC97 noise during voice recording

Recently I installed a new motherboard, and pretty much everything is working except voice recording from my headset (I use Ventrilo and TS a fair bit.)

Here's a sample of an open mic using Windows sound recorder:

http://boomp3.com/m/97979753544d

As you can hear, there's a continuous sort of oscillating noise in the background. I half wondered if it could somehow be interference from one of the three fans in the box. Thing is, every other aspect of sound works fine (desktop sounds, game sounds, MP3/AVI playback). It's only voice recording that has this problem. I'm thinking IRQ conflict perhaps but I'm not really sure how to proceed. Any advice gratefully received.

I have:

1. Flashed the BIOS to the latest version available on DFI's site.
2. Installed the latest Realtek AC97 drivers
3. Completely updated Windows via Microsoft Update, and installed the latest DirectX
4. Installed all peripheral drivers. There wasn't one for my headset, so I'm assuming it doesn't need one.

Motherboard: DFI Infinity RS482 with Athlon 64 3700+
Onboard sound: Realtek AC97 ALC850
Headset: Logitech Premium Stereo Headset (problem also occurs with a Sennheiser PC130)
Windows XP SP2

 

Just the one bump, then I'll leave it. No-one have any bright ideas (apart from "buy a soundcard", which is the next step)?

 

This may not be it but try...
Open your volume control and mute the cd-rom.
Ive seen lots of instances where it will cause buzzing just from moving the mouse around on the desktop.
Krank up your software/master and your speakers volume and move the mouse around to see if you get interfearence/buzzing... if so try this tip.

 

Yeah, CD is already muted (as is SW Synth, Line In, Microphone, and PC Speaker.)

Thanks anyway!