The old floor was hardwood suspended 1.5" above the floor on 2x2 "sleepers" with no subfloor. Since the floor was floating relying on the t&g to hold it up, all the boards move=squeak.
Sorry sideways but shows the old floor with part removed.
We removed the old floor and sleepers, put down a proper T&G subfloor over the original subfloor after nailing that down. We nailed new 3/4" hardwood to the new subfloor.
Plywood has tripled in price or we would have added two layers so to save $1000 material $600 labor on a $7000 job we lowered the floor.
Also: part of the house they already had replaced the hardwood with interlocking floor where they had doubled the subfloor to match the height; I was able to pull the top layer from two rooms and save$150 in material by re using it.