is yours a manual. have you tried removing the rubber bung in the firewall on the passenger side footwell and then drive it to hear the noise. only reason I ask is because when I did some work installing some bits and pieces with the bung removed I noticed the same noise. its quiet untill 2800 then it rattles sounding like ping but in neutral with no load, then after 3200 the rattle stops. also revving it to 3500 in gear then on the clutch in for a change to second u hear the rattle, on every gear change. I dont know what it is but its worrying me too.