Not sure if this was the case with 13.04 as well but I've only noticed it since a week ago or so. The fans aren't terribly loud and when I have my headphones on I don't even notice but it's really weird and can't possibly be right like that.
Every minute or so (sometimes more frequently) the fans turn up for around 10-15 seconds or so and then off again.
Without me doing anything that would require that. Any idea what to do?