High pitched whistle. 97 Taurus

High pitched whistle, but only occasionally.
It happens about a quarter of the time. It whistles when I accelerate and slow down. If I get on the freeway, it goes away completely for the remaining drive time. Any suggestions?

I had this problem with my 96 Sable and it was the belt tensioner idler bearing. I also had this same sympton with my 90 Taurus and with this one it was the alternator bearings. Both were cheap fixes.