having engine problems with my honda engined rover 620sli 1998?

I have had the car for about 6 months and it seems to have suddenly develloped a fault that i cannot diagnose, When idling the engine runs fine with no strange noises or rattles, no problem, But when accelerating / reving the honda engine, it makes a sound like two small pieces of metal hitting each other, almost like rustling tin foil if you like. However i cannot see any problems with any moving parts, it sounds almost like it is coming from the cat converter?? The engine is however just due for a cam belt change? Anyone have any ideas what it might be and if its very serious? i know a fair bit about engines and can probably fix problem if i know what it is! Any suggestions will be very greatfully recieved! Thanks in advance! Jason.