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Spray talcum powder in the grooves of the hardwood floor. Try to have it penetrate in with a brush, and add some more from time to time. This should help lessen the "squeaky" noise.
Source: Common used knowledge in France
By Olga from Miallet - France
Locate which plank is squeaking and sprinkle some talcum powder in the cracks. Let it sit for a while so the talcum powder can work itself between the cracks.