At altitude and at higher power settings may be a leaking pneumatic or muscle line in the interstage area. The 9th stage may be overcoming the shutoff and whistling. A very small leak will be VERY loud at altitude and since it could be in the pylon or very near to it the Pax will be getting the best benefit of the malfunction in their auric canals. (wow I didn't know I even knew that word)
You won't see a pressure problem on the overhead as the PRSOV should take care of that small matter.
They should be able to duplicate the leak on the ground but the noise won't be as noticable because the fan is making its own noise in sea level air.
I've heard the difference between a CFM at power at sea level and one in Ft. Collins. It's pronounced.
Lots of luck on this one.
Let me know
