Originally Posted by Papa2Charlie
The most likely cause are HP bleed valve(s) without silencers being open during the start. As the engine reaches idle, the valve will close and the noise levels will drop significantly.
The HP valve is definitely not closed at idle. ....Given that you're evidently not too familiar with the beasts, I'm a little curious as to how it is you feel you can say what "the most likely cause" is? HP bleed valve "silencers"?? Never run across one on a GE or a Pratt? ....could that be one of the odd features of those RR (or should that be VW?) makes?
In any case, the noise I've been talking about, and which I took the question to be about, is not an air related noise at all.... it's an ugly sounding vibration. Maybe I'm in left field though.... wouldn't be the first time.