I would also guess it's the steel wheels flexing. The Lincoln Town Car I used to have had a creaking noise that I couldn't track down, I thought it was ball joints or a-arm bushings, so I replaced all the front end bushings and it was still there. I forget how I figured it out but it turned out the noise was from the aluminum center caps on the wheels. I just rolled without the centers after that, but a dab of grease probably would've worked.