Noise driving me mad, help please?

Nissan Tino, 6yrs old has begun to frequently make a very loud clattering sound, sometimes when climbing a small hill, sometimes on the flat, today when idling. Have had exhaust, heat shield, mountings, etc checked but nothing to see. Dealer drove it arounf says not gear box, clutch or wheel bearings. Today could have sworn I was trailing metal under the engine, Please help.