i just picked up a 2003 1.8 audi T Sports cabriolet... its all good expect there is a knocking/rattle noise coming from the part where the roof meets the windscreen.. its more to the passengers side & happens when driving in London going over uneven road surface or a drain plate/small pot hole or even a road hump..what could it be? what can i do?

info really appreciated as worrying me...

thanks