97 passat... strange noise around 30mph - 70mph?

Hi !... I've recently bought a 97 ( R reg ) passat. The car drives wonderfully but, I've suddenly noticed a repetitive noise that i can only describe as a playing card rubbing through the spokes of a wheel at around 30mph - 70mph, sounding a lot more dull than tapping... i'm worried about it as every time i take it the garage the noise vanishes. If anyone has had this problem and solved it or if anyone can help, i'd be very greatfull... Thank you !