2000 Ford Taurus What could be cause of the noise

I bought the car 6 years ago with 100k freeway miles and my mechanic said the 3.0 engine was great and could go another 100k. I keep it well maintained & serviced and only a week or so ago I noticed a rather loud high pitched whine when accelerating or decelerating. Mechanic took it for a drive and said it sounded normal to him but of course it wasn't very loud when he drove it around the block. On a 2 hr freeway drive my passenger even noticed it. I just want to know what a possible cause for this noise could be.