Originally Posted by Versus_Creations

there should be an air leak from the bottom of the throttle body which is controlled by a screw called. The idle screw. By adjusting this it will change the air leak amount and hence change the RPM. It will also effect the AFR which is why you shouldn't adjust anything unless you know what you are doing. Next is that air flowing through the manifold itself and pipework etc will also causing a slight gushing noise to make the sounds even harder to distinguish.

Chances are without knowing anything else other than its MFi, it will need a test of components setup tune and road adjustments after.

Take it to a specialist that will charge reasonable money to have this done. Don't just go to Bobs mechanics round the corner as they prob don't know enough to get the most out of your system.