2000 Ford Contour O/D light flashing?

Hello; I recently purchased a 2000 Ford Contour with only 32,000 miles on it, one owner. It is a gem (confirmed by my mechanic). I was driving tonight and my O/D light started flashing, steadily for approximately 10 miles until I got home and turned off the car. I noticed no difference in driving or shifting, drives great. I looked in the owner's manual, it says it means there's a problem with the transaxle. What could possibly be wrong with it? Of course I'll take it to my mechanic to look at, but any idea? Thanks