I've found many onboard sound cards to do very odd things. We keep some cheap PCI sound cards around when we suspect the onboard sound card or drivers to have less than stellar performances.
I'm running Win2K Pro on a Compaq SP750 with dual Xenon processors. When I received this computer, I wiped the HD and installed Win2K from an MS Original CD. Ever since I started using this computer about seven month ago, at random intervals (typically about six to ten times during a seventeen hour period) a sound that can best be described as that which would be made by a cat whose tail had just been trodden on, will emanate from the speaker. It can happen whether or not I am using the computer, as long as it is turned on. I have searched for *.wav files and played every one of them, but the sound isn't there.
Any suggestions on finding the source of this sound would be appreciated.