I can only speak from experience of 74s. When at ground idle, standing by the NLG and talking to the Flightdeck,it's pretty obvious the engines are only at ground idle,you can clearly see the white spiral on the spinner fairing rotating.Thats why it is there,in a very noisy environment you need a visual indication the engines are running. Once the engines start to accelerate,it does not take long for that indication to vanish,in fact at high thrust you can clearly see the intake guide vanes on the core. When taking part in a high power test run,especially looking for the start of a surge,you would be a good distance in front of the NLG,even with intake guards in place.