1999 Peugeot 106 strange flapping noise

Right, please bear with me. I bought the car last Saturday. I took it for a test drive before handing over the cash. heard nothing! I ddrove it home the 35 miles and it all seemed well. The following day however, I heard a ringing noise, which soon brought on what i can only describe as flapping. The ringing noise I fixed as i pulled off the wheel trim and the little circle disc covering on the wheel hub thing had come out. I pushed that back in and no more ringing. For the life of me i cant find this flapping noise. Ive jacked the car up, touched everything under the car, nothing hanging or loose. No good with engines so not looked in there, but the noise can only be heard when i have the passenger front window open. It gets faster the faster i drive, and only stops when i go round a roundabout or reverse. I have had CV joints and its defo not that. Has anyone had the same problem? It makes my sexy little car sound like a chavvy car and it makes me not want to drive it.