filling gaps under laminate floor boards?

Hi all...I have a KronoSwiss Noblesse laminate floor installed over wood joists/plywood. Unfortunately, there is one high spot in the subfloor so there is a flex in the floor for about 2' around that high point. Nothing terrible to me, but I'd like to fix it if possible before/if any joint problems arise. Pulling up the entire floor is not an option at this time.

I was thinking of filling that void below the laminate by core drilling a couple of small holes in the subfloor from below and pumping in something to fill the gap. I have had a few things suggested to me:

1) self levelling grout - this sounds good but I am concerned with the moisture coming through the floor and damaging it.
2) expanding foam insulation (shoot some in, let it expand and fill the gap, then harden). Also sounds good but will it push the floor up? Will the chemicals damage the floor?

Anyway, just wondering if these might work or if there is another product or method I could use to fill that space under the laminate if possible. Thanks!!