I have a 2007 Expedition with the built in navigation system. Vehicle has about 120,000 miles. Lately there has been a clicking noise coming from the radio unit. The noise is not related to the CD changer but instead the DVD unit for the navigation system. The sound is similar to hard drive that has gone bad on a computer. The sound will sometimes go away and then return. Today after filling up with gas the sound stopped but got a message on the screen that the Navigation DVD may be in operable and to eject and clean or replace. I did eject and clean the DVD but the noise is still there. I'm thinking if it was a scratched DVD the noise would be constant and not go away periodically. Is there a way to clean the drive unit? Anyone else encounter this? The navigation system itself still operates properly despite the noise.