If the tops of the floor joist are not flat and level then the sheets of plywood or sub flooring will not be fully supported evenly.
Plywood floor squeaky in my new house.
For squeaky floor repair begin from underneath the floor if you can.
Squeaks happen when a house settles and wood flooring dries and then expands.
Sub floor is osb tongue and groove 3 4 on upper floor with carpet main floor sub floor is osb tongue and groove 3 4 on lower floor with 3 4 hardwood.
When nailed down the sub flooring will be sucked down unevenly.
Squeaky floors may alert you to kids sneaking in past curfew but they ve got little else going for them.
Since day 0 one of the major issue had been my flooring both main level and the upper level.
Over time the portions of the floor that have been sucked down will try to go back to a flat and level position.
Annoying floor squeaks common in many homes typically occur after the house has settled and flooring lumber has dried out and shrunk.
I have some squeaks in my floor that i have narrowed down to exposed nails coming through the plywood sub floor from the basement which is not finished.
Nails rub on plywood when walking on floor and cause the squeak.
Separate wooden bridging members to eliminate noise problems.
Before i attempt to try anything i thought i would see what you suggested.
I have posted the video below of the upper level.
This old house general contractor tom silva silences some squeaky floors.
This causes the floorboards to rub against each other or against the subfloor or against the nail casings.
See below for a shopping list and tools subscribe to this old house.
If the underside is covered you will have to work from above.