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.
Over the weekend I went a bit solder happy, meant to change just a few lights but I've changed all the lights in the dash, headunit ( took 2hrs to do the f'ing thing ) and even the dials and trip lights.
I've made up a little guide on how to do etc but it's ended up at 37MB and I can't post it, I still can't even quote on this site for some reason.
Is it is over interest is it possible to mail it over to someone to post up on the site?
Gone all type-r and have all deep red all over bar the sat nav, looks pretty boss.