2001 Renault Scenic ticking/rattling when i rev the engine

when i rev my engine ,just lightly ,i am getting a really fast rattling /ticking noise ,the harder i rev the faster it gets . this is only happening when i rev the engine,i have been in the engine bay to have a look and the noise is coming from around the alternator part of the engine but i cant pin point just where it is coming from ,my father inlaw had a listen and said he would not worry but to get the t/belt changed ,but typically when he heard it it was not doing it so loud. I have no idea what this could be and its driving me nuts ,and im scared to go far in it just in case something happens. P.S only when i rev /not when idling .THANKS VERY MUCH