vito wind noise

hi all, ive had the vito 108cdi 2001, from new and have had trouble with wind noise coming from top centre of windscreen area, only happens on speeds of 60mph and above normally on windy days. the windscreen is original. if i put upward pressure on top of screen this sometimes stops it sometimes not, i do have roof rails fitted but do not know if this has any bearing on the problem. if anyone could help i would be grateful. Harty