I have a 98 Expo that was making sounds like the Serpetine Belt was slipping at 3500 Rpm's and above. I purchased a new belt and replaced the factory belt (50,000 Miles). The next day the same sounds returned so I took it to the Ford Dealer thinking it could be a Tensioner Pully. At Ford I was informed that the new belt was the wrong belt. After comparing the two belts the new one was about 1/2" longer. (I figure the automatic tensioner should adjust for that little of difference) Anyway Ford replaced the belt (Total $146.00). After a road test I was informed that the problem was not corrected. I was then informed that a water flow restrictor would have to be placed in the heater hose to restrict the flow of water to the heater core since "they think" that was causing the fins in the heater core to vibrate and make the screeming sound. (Another $110.00). Vehicle is still under extended warranty which would lead me to believe if the heater core is bad...... replace it under warranty..
Has anyone ever heard of this problem? I would appreciate your reply since ford wants me to bring the vehicle back to them and try again.
BobE

 

For searching purposes though I would add to this thread.

I have just had a similar complaint on my 2000 Expy EB solved by the recommended procedure. I was getting what I thought was a transmission whining/moaning noise at about 2500-3000 RPM. Could be heard in the cabin fairly easily and had been getting steadily worse Adding a water restrictor in the heater hoses cured the problem.

I suspect its a high speed flow of water in and out of the core causing the sound in some manner.

Eric