Your going to get at least 6 differant ansewers on this one question, it's a hit or miss thing and anyone of the suggestions may or may not work for you and how your floor was laid. Here's just one idea. Stop Floor Squeaks Carpeted Floors Hardwood Floors
It could be something as simple as the house is to dry so the wood has shrunk up and has room to move. Buy a cheap humidity gauge and see what it says, it should be around 45%.
Thanks for the information I was looking around for ways to stop the hard wood floor in my guest bedroom. It is always annoying when you have people over as you can hear them walking around.