Slight flutter noise

Whilst travelling back from Whitby yesterday I thought I noticed a faint fluttering noise from the rear. The noise was more noticeable though very faint when traveling at 80 mph on the motorway. Roof down of course. It did cross my mind that it may have been the wind buffering off the central reservation barriers. Checked the rear wheels tonight for any wobble or side to side movement and both seem ok. Any ideas guys?

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