Strange noise when cars pass me!

Hey guys, bit of a strange one. I haven't had my S3 for long so haven't really been out for a blast until today. I've noticed that when I drive along fast B roads and cars pass me in the opposite direction there is a clicking noise coming from the rear of my car! It's only if the car passing me is moving at speed creating a blast of wind. I thought I must have left my petrol flap open but it wasn't. Could it just be the change in air pressure opening and closing the flap as the other cars pass? Or a panel flexing? It is really annoying so any advice would be great.

FYI it's a 3 door S3.

Smiffi

Ok, so this obviously isn't a wide spread thing. I've just checked my petrol flap and it is very easy to open it by pulling on the flap rather than pushing it so it pops out. It does lock when the car is locked. I wonder if the hinge is too loose for the flap so when I’m passing other cars the flap is getting sucked open slightly then slamming shut. I can't think of what else would make a clicking / slapping noise as a car passes in the opposite direction.

Really, any thoughts welcome. If someone could listen out for a noise the next time they are passing fast moving traffic in the opposite direction it would be much appreciated.

Smiffi

Cheers guys.

Its defo not a stone though. One thing I'm thinking is that when the car is locked, the solenoid moves across to lock the petrol flap. So I would think that when the car locks itself when we are driving it must lock the petrol flap? If this isn't happening, it wouldn't take much to move the flap as cars pass causing that vacuum effect.

Does anyone know if the petrol flap should lock when the car is on the move and the auto locks are initiated?