I have to support Papa2Charlie's contribution here, however he had the HP bleed valve confused with engine handling bleeds/start bleeds!
It is very obvious, that the deep 'awesome' vibration sounding resonance, experienced during engine start/spool-up, distinctly shuts off, disappears, as the engine reaches steady state idle, which can only result from some sudden change, which is the appropriate engine handling bleeds closing, hence silencing the aweful rumble, eminating from the mass compressor air mismatch of N1 to N2 airflow, during sub idle/operating RPM's.
An engine of these proportions, will probably never reach max motoring spec's without all the handling bleeds wide open, dumping the excessive air and unloading the starter turbine, let alone accelerate, once fuel burn is induced.
Once the GE90 reaches steadystate idle, it rapidly becomes incredibly quiet. Smooth, matched, controlled and sealed compressor airflow.
I agree with 3holelover about his 'bowed rotor effect' on GE engines, however this pertained specifically to the CF6-50/80's, which had very skinny & long LP shafts.
I doubt this phenomenon is creating the majority of the loud audible & physical rumble that every GE90 delivers on almost every engine start.
That kind of vibration/resonance being the result of a long shaft whipping within its bearings will no doubt have caused GE to issue an immediate AD to force 20 min cool-downs, as it would seriously shorten engine reliability and life.