Trying to analyze and repair a squeak in a beautifully finished hardwood floor that's inaccessible from down below because it's located on the second floor can be tricky. In theory, floors squeak when boards move up and down on a nail or rub against each other when weight is applied. The first thing to diagnose is the type of squeak you have. Is it a whole section of floor that has loosened? If so, locate the joists below with a stud finder; drill pilot holes in at a 45-degree angle; drive 2-1/2 inch finish nails into the joists; and putty over them. If two boards are rubbing together, try "lubricating" them with talcum powder, melted wax or graphite. If that fails, angled nailing might work. Be sure to nail into a joist.