Citroen Xsara rattling

Hi everyone. I've had my car for three weeks. Bought secondhand from someone who had it from new. It's well looked after also. BUT there,s a noise which is infuriating me to the point where I don't want to drive it. I can't make out where its coming from, although its nearer the front, but not in the engine. Def not at the back of the car. All wheels checked and in good order. It honestly sounds like a stone rattling around in the exshaust. Is that possible...it's driving me nuts! Anyone else have issues like this, what else could it be? Thanks in advance!!