Pwwwwwoo noise

[MacHiavelli]

24 inch iMac running XP under boot camp.

Got all the sounds turned off, but the iMac keeps making a pwwooo noise (like something out of space invaders or a kid making the noise of a bullet whizzing by). If I stream a radio connection, every word spoken has this Pwwwwwoo Pwwwwwoo noise after it. Any ideas? As I say, all sounds are turned off in the control panel. Even clicking F1 F2 to change the brightness causes the noise.

Thanks

[MacHiavelli]

I downloaded R1.85 drivers from RealTek and the problem has disappeared:

http://www.realtek.com.tw/downloads/downloadsView.aspx?Langid=1&PNid=14&PFid=24&Level=4&Conn=3&DownTypeID=3&GetDown=false