1998 Ford Explorer

Transmission problem
1998 Ford Explorer 6 cyl Four Wheel Drive Automatic 139000 miles

When I am driving Periodicly u will hear a sound that sounds like someone is sticking a pencil into a fan blade. It does it most noticeably at around 30 M.P.H and above. When it does make this noise it may last a second or it may last 15 seconds or more. I have taken it to some local mechanic's and none have heard a noise like this, At first I thought it was the drive shaft, but that is tight and appears to be fine. The last shop I toook it to thinks it may be int he transfer case or the transmission. Any one else had anything like this?
October 28, 2010.

Does the sound seem to come from under the vehicle or under the hood?

Under the vehicle, when driving it sounds like it is in the center under the vehicle.

Oct 28, 2010.
Have you checked the splines on the driveshaft yoke? Also, did they check u-joints? Is there any chance the 4wd could be trying to to engage.

The Shop I took it too, Said they checked both the drive shaft and U joint's. They said both were tight, I had some other people tell me to have those checked as well, As for the 4 wheel drive i, m not sure on this. It is Auto 4 wheel drive at all times then can be switched from 4 high to 4 low. Another thing I remember is back In june or July I was driving at about 25 m.P.H when I heard a squeel and a pop/thud from underneith the truck, I immedietly pulled over but found nothing wrong. After that it was a few weeks before I statred hearing this sound

Oct 29, 2010.
This is a tough one. Based on your description and what the shops have checked, something tells me it is in the transfer case.