As stairs are only made up of three pieces, stringer (which is the side supports), the riser (which is the facing between the treads) and the tread (which is the piece you step on), the noise must come from either loose nails holding them together or movement between the pieces.
If there is a gap between the stringer and the treads you can use either wedges or you can insert a new piece of wood to the stringer (glue and screw the wood to the stringer) to stop the movement.  Sometimes just nailing down the treads again just doesn’t stop the …read more