Background noise when slowing down on videogames ?

Hi !

I've a little problem, and I don't know very well if it's the
motherboard that is wrong, or if it depends of somes settings of the
bios...Maybe somebody can help here ?

Config :
MSI-6309 (VIA chipset), with a celeron 766
On it, Geforce 3 Ti 200, and SoundBlaster Live 1024

On some games, when frame rate go low, I have a parasite background
noise....
I've changed the soundcard with a SB128, same problem
I've changed the videocard to GF4 MX440, same problem

I get this problem on some games (Elite Force 2, and Silent Hill 3), and
not on some others (Unreal 2)

When the background noise start, il will progressivly disappear when
frame rate go fast...

Any idea ?

Thanks.