I bought this house few months a ago. This home is almost 13 years old so there are no squeaks through out the flooring except on the second floor between the dividing wall of two bedrooms. What I have noticed is that the floor squeaks close to the dividing wall on both sides. The wooden plank goes under the wall and is in both rooms. If I walk on the floor near the wall, at some places I can actually see the floor moving. I noticed several joists between the gaps of sub flooring and I put several screws in line with the existing ones. The problem is still there. Please note since this is the second floor, there is a finished ceiling beneath. All the solutions avaialble on different forums talk about fixing the floor under an unfinished surface. Any help will be appreciated.

Thanks