i've just bought a 1999 expedition, the first thing i noticed when i drove it on the free way was that it made a constant sound especially after i started to speed up. Its more of a loud humming sound, It only makes that sound when its a straight road but as soon as i make a turn on the freeway it stops. and the noise picks back up when im back on a straight road. Maybe someone can tell me what the cause of it is, please