Maybe a browser Window, Internet Site which reloads every few seconds... Default sound is a 'POP' for this... but rather silent..
Sounds like a software issue to me. Maybe there's some specialized software for this, I don't know. Task manager will show you the names of the executables that are running at that moment. If there's not too many, you can check them all. Tedious, but doable. Also, it could be an interference from other objects. My speakers (very low quality) start to buzz if I leave the cell phone very close to them.