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