I have a 200 Eddie Bauer Expedition. It squeaks terribly when driven with most of the noise coming from the front. It sounds like an old spring mattress. It makes the noise when going straight or turning. I think the power steering might also be making some noise as well, but it is hard to tell
More than likely your squeak noise is coming form the lower ball joints. This is a common failure part for the ford trucks/suv's. Have both replaced and the noise should no longer be an issue. With the vehicle parked, you can push up on the door frame in order to get the vehicle rocking and the noise will occur. This is a clear sign of the lower ball joint failing.
If the noise changed when turning I would say it might be a Hub Bearing