Some a/c have an APU fuel pump that sits in the falsework in this area, but wouldn't account for the noise change with N1. Or the triangular panel over the landing gear bay, I've found a few missing fasteners in these before and I know our company removes them alot.Engine hydraulic shut-off valve is also in this area but you'd probably have other symptoms/warnings with this. Or there's the possibility the gear door(s) aren't faired properly, it's not something that would be immediately visible on the ground. As already mentioned, an aircon snag of some sort, pack valve maybe.
Just a few thoughts
Eng
