Is the undertray rear fixings secure?
Sounds like it maybe the undertray flapping once at speed.
Just a thought
Urm not 100% sure where you mean?
I just took it to a mate who works at a garage, had a quick drive and he seems to think that the top of the struts are loose - so over the week as the vibrations have made it worse. Sounds like a pretty good stab to me, will get it taken somewhere on Tuesday and ask them to make sure everything is secure.