louder noise and less power.help?

recently out on my gilera dna 50 and suddenly the engine become louder and the speed decreased.i had noticed that the bike was losing some power the past week before this happened bt didnt think much of it.can anybdy help with giving me a diagnosis of what they think it is as the mechanic looking at it has had it for almost a week and hasnt even started to strip the engine to have a better look.

iain
October 2007