Music and Video Slow Down

Hi when I play music and video there will be an annoying digital slow down causing sort of a "whaaa" sound (i don't know how to describe it) when the cpu is being used. I can make it sound its worse when I run dxdiag. It happens on all media players and regardless of ram/cpu speed. My specs are: Tbred 1700, Shuttle AN35N, 512 HyperX PC3000, 2 IBM 75GXP's in Raid 0, Game Theater XP, Win XP Pro (fairly new install, but this issue started a while ago and i don't know what caused it)

Thanks

anyone ever experience this?