The IAC is open while the engine is warming up, once it closes, you don't hear it anymore. If you did still hear it after the engine was warm and running for a minute, you'd have a vacuum leak. The IAC is basically a computer controlled vacuum leak that still leaks in filtered air. You could build an air box around your filter to help hide the sound, but that can get pretty pricey. Other than that, there is not much you can do.