The engine has hydraulic tappets, they can fail to "pump up" on start to close the clearance between the rocker arm and valve. They usually pump up and quieten down after a few seconds running but I suppose they can fail completely and rattle. Maybe worth running for a short time at idle with the rocker cover off to see if that is where the problem is?
That is good advice as I was wondering if that was possible. I presume that by running it for a couple of minutes and being careful to make sure it sits ok on the seal when putting back together all should be fine?
