Imagination or an actual sound

[fiestalux]

Hello,

I have a 2014 Fiesta Ecoboost. Recently, I have started to notice what sounds like a sort of rotating metal sound when driving at higher speeds on the motorway. It is kind of a middle pitched sound rather than a rattling etc. The sound is not there when a lower speeds. Is this common and am I just imaging its new when it was in fact always there? If my hearing is correct then it seems to come from the back of the vehicle, probably on the right.

The service is due in the next few months, so perhaps it can be examined then.