Is it engine or road speed related?if you coast engine off down a hill for a few seconds does it still rattle? If so might be bearings CV joints or similar.

If memory serves, the 1.8 petrol is the Mitsubishi GDI engine, they can pink / detonate quite a bit when accelerating as they're a very lean-burn design. This makes a pronounced, rapid metallic tapping / rattling sound.

When the rattling starts, push the clutch in and take your foot off the throttle to let the car coast for a second or two. If the rattling stops, it's probably pinking. If the rattling continues, it's a wheel bearing / suspension component issue.

If it is the engine, first thing to try is filling up with a premium 97 / 98 octane fuel (V-Power, Ultimate, etc) as I think these engines were supposed to use high-octane brew. That might stop the pinking.