well not having underneath access makes your easy job much more difficult.
What is happening is that the treads were fastened to the stringers with 8 D finish nails. The wood around the finish nail has loosened up and when you walk on that tread it is bouncing up and down on the shank of that nail which in turn is making the squeak.
being able to screw the stringer to the tread from underneath will solve the problem, but in your case any above work might damage the look and appearance of the tread and stairs. So, some WD-40 on the nail area or a drop of oil on that nail will stop the squeak for you, you could also screw the tread to the stringer and then counter sink the screw head and putty over that and then either stain or paint and it will also be permanetly fixed.