An engine will make that "vroom, vroom’ sound when revved because it is moving more air through it at a varied rate depending on how much air is let in by the throttle. This much movement of air and it’s variation causes noise and vibration, resulting in that familiar “vroom, vroom” sound. If you are experiencing more noise (louder “vroom, vroom” than usual), you probably need to have your exhaust system inspected. The exhaust system does a lot to keep that “vroom, vroom” noise to a minimum.