In my dinning room there is a sag. A support beam was put in place years ago but no one corrected the floors. when I step on that area I can feel all the hardwood sift around and it almost feels like the floor is soft in the spot. Makes a horrible squeaking/cracking noise.

In the basement I can see I have the old plank subflooring. Is this the issue the subflooring and if so is there a way to correct it from under rather than ripping up the beautiful 80 year old oak floors to lay new subflooring...

Also I want to fix some squeaky areas... any ideas... Like I said I have access to the subfloor and joist in the basement.