THe engine at 75K has suddently produced a rattling noise when coolant temp reaches 80-85. This is heard inside the salloon and dissapers when engine in ramped up and comes back again once rpms at 750. Nothing to do with gearbox, as depressing the cluth pedal changes nothing. It's not the noise from the valves etc. as all those noises are still there as usual. Any idea what it could be?