When engaged, the Cat A switch (i.e. the 3% nr increase) actually makes the aircraft noisier. The threshold at which it is deselected is due to noise abatement considerations. The combination of the higher nr combined with airspeeds in excess of 55 kts cause the EC135 noise signature to exceed the level at which the aircraft was certificated....
...at least that's what the bloke down the pub told me.