Are you doing anything in particular when you hear the noises?
Have you just closed your browser or another program?
Is there anything at all you can think of that you do or have done when this starts?
I can honestly say I have never come accross this before, The closest thing I can remember was a client with a tv/radio tuner card that would unexpectedly play television.
I would suggest going to www.ewido.com and running their free online spyware scan to se if you have missed anything with your previous attempts.
Other than that do you ever use real media or listen to any online radio stations?
The problem as Eric suggested is most likely caused by spyware, which may have installed itself on you computer. It can also be some kind of live background audio coming from certain sites when you visit them.
If the sound is only coming when you visit certain sites you can easily determine which site is causing it by monitoring the open websites at the time the problem occurs.
If the sound is not necessarily coming from one of the visited sites but due to some spyware installed on your system, you can find out the program that is causing it as follows:
Next time you hear the sound, click CTRL + ALT + DEL keys to open the Task Manager. Then click on the Processes tab, and end the process of programs that you do not recognize, one a time, until the sound stops playing (be careful not to touch explore.exe) . That last one you have ended would be the culprit.
Hope this helps.
Did the removal of the extra spyware take care of your audio problem?
Please let me know and there should be an accept button you can use on this answer if you are satisfied with the information I provided.