I have a very squeaky floor and it is because the hardwood has voids between it and the sub floor. I am about to install a saddle in a bathroom renovation and I now have a cross sectional view under the floor. At this time is there anything I can do to inject something into the void before I put the saddle in. I am not worried about the saddle as that will be set in firm reinforced substrate, but before I put that in I have this little window. I was thinking I could stop some the squeaking for at least the area I can get to from this opening. I don't expect to be able to solve the entire problem, but I would be happy to do what I can do. The voids were caused by some settle a long time ago which has stopped. So any recommendations would be helpful. I was thinking of injecting some caulk or thinset?? Just to limit some of the movement. What are your ideas.

Not sure how we can see what's going on here but the noise isn't from the voids it from the movement of your flooring, voids just let the deflection get worse. How did you get void, the hardwood should have been nailed directly, firmly into the sub.

It's a floating oak hardwood floor touge and groove. The voids were created as the house settled over the concrete retaining wall there is a little hump there and the created voides on either side of the hump. The house is 40 years old and this process has stopped.