Ford Expedition (Ear Ringing Sound)

I have a Ford Expedition with 35,000 miles 2004 and when we hit 1800-1900 RPM around 60 mph we get this whine (sounds like when your ears ring). It seems to be coming from the front dash area. It ususally happens 15 minutes after the vehicle is driven. Sometimes it is loud, other times it is very faint. It is intermittent. It does not happed all the time. When we take our foot off the accelator it usually stops. It is so annoying and I took it to my dealership and they could not replicate it. So I was told that it would have to get much worse before they could fix it.

I even called Ford corporate - nothing.

Any ideas?
Thanks.