Floors squeak over time as things settle. If you're wanting to take measures in to your own hands, there's this neat little kit called 'Squeak no more' that they sell at lowes, home depot, an online. It's an awesome kit that solved a few squeaking issues upstairs in my house. On a wood floor, you just drill a small pilot hole, then it has a plastic plate that you put a screw in, screw it down, and the screw breaks at a certain spot leaving only a tiny hole to fill in with putty. Works well through linoleum as well, and has another adapter for carpeted floors.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry