I've had something similar on my two fiesta's, not so much my newer one.
The only thing that could replicate it was reaching 3k rpm in 2nd or 3rd gear and a little squeak would crawl out near the drivers side wheel. Was very high pitched. Did this on flat roads, cobbles (Don't try this) straight roads, bendy roads etc. sometimes it did it, sometimes it didn't.
Just got very annoying thinking that something was wrong, Ford checked, nothing, another garage checked, nothing.
So yeah, take that as you may.