Sounds familiar.
I had no issues until I started taking the Fiesta to the track (road courses) at about 30k miles.
My first trans developed a loud whine in 4th and was replaced under warranty.
My second trans developed a loud whine in 4th and a moderate whine in 6th and was replaced under warranty.
My third trans started to develop a whine in 4th, but by this time I had figured out how to keep it from getting worse by not shifting mid-corner. After a bit I had the entire transaxle rebuilt locally to figure out what the hell was going on. The rebuilder theorized that the dual-mass flywheel either lost a spring or just couldn't keep up with how I was doing mid-corner shifting. I had already changed my shift points, so I had a single-mass Spec flywheel installed, but I stopped taking the Fiesta to the track before I really tested the theory.
I never lost a gear in all of that, the whine just got unbearable to drive with.
FWIW, the other people I met at the track with Fiesta STs haven't had this issue.
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