I have a stairway going from the first floor to the second floor. The stairs go straight up then make a left turn on two triangular landings, then two more steps to the hallway on the second floor.
The two triangular landings are what is squeeking. they are made of wooden strips of oak 2 inches wide. It seems that the strips are giving, when you step on them, and I believe they are rubbing together, making the squeeking sound.
I do have underneath access to them in a closet. However the closet is closed off with a drywall ceiling where the landings are located. Is there any way to silence them without having to take the landings apart?