Let me just clarify something here....
When I said a HP Bleed Valve, I wasn't speaking about cabin bleed. I mean handling bleed valve(s) in the HPC. At low N2, the HPC BV's will be wide open to help stabilise the compressor airflow. As the engine nears idle then the offending BV will close. The remaining valves on the HPC will open / close as usual during accels / decels. Without knowing the details of the booster arrangement on the GE90, it's possible the noise could come from an LPC BV also but my intuition would say the HPC is more marginal on stability during engine start.
I don't know the in's and out's of the GE90-115B but from my own experience of hearing a couple of them start, the noise is consistent with a handling BV being open at start then closing once the engine nears ground idle.