While driving at highway speed, I get a sudden loud grinding sound that lasts for about 20-30 sec. then fades away. It sounds like a fan motor bearing or a speedometer cable that's wearing out. The sound is very intermittent. Its location appears to be from behind the dashboard on the driver's side to the center console- certainly in the front half of the car. It is speed sensitive- it will gradually disappear as you slow below 35 mph. I've tried turning off heat/AC controls and switching from 2WD to 4WD- no help. If you coast at 55 mph in N and gradually increase rpm to 3500, it will dissipate somewhat, then suddenly stop as you shift back to D (most of the time).

Any ideas? Thanks for your help!
Bob