My 1999 ST24 Mondeo has the same problem, i can hear the motor running but the seat doesn't move.
Lukily it go stuck full down. I have had a good look under the seat and taken a few plactic bits off, but the motor looks a sealed unit as part f the seat base .
So i'm interested also on the cost effective fix for this problem, must be something loose in the motor/drive mechanics. ??