Strange Noise On Standing Still?

[vernall]

hi guys,had my 1.4 tdci (60 plate) for a little over a fortnight,since its been nice weather have had the windows down and noticed while car is stood at traffic lights for example(without any acceleration) that the car makes what i would describe as quite a rattly sound,as soon as you take off its sounds fine,but may be there but i cant hear because the cars moving.had a mate in the car with me the other day,and was sat at lights and he said is that your car making that noise.dont know if its always been there as wouldnt of had the windows down before to notice it,but i can hear it with windows up now,but that may be because i am listening for it lol

any ideas please?