Squeaky floor above studwall

Danny | July 29 2021 | LS9

Converting a house into 2 small flats. It the sound will be annoying when tenants are sleeping below. So In the attic bedroom (5m x 5m) the floor is squeaking in the centre area. Can be heard and annoying. Under the floor is a stud wall which was done about 15 years ago. The noise is only from the centre 4 joists. Do I need to install more togs which I started doing but didn’t work? I can’t see if there’s a small gap below the joist as the stud was done under the ceiling with plasterboard. I think when walking the floor is grinding on the top of the studwall. Been told to try talc powder. Screwed in all the floorboard to the joists. Iv tried screwing from above plasterboard to the studwall. Any advice will be helpful.