I tend to agree with Muskrats suggestion that it could be a loose valve guide. In fact I think that a loose guide is the most likely cause, and the reason that I say this is because you say that the noise is eminating from the top on the left hand side. Often you will see evidence of oil burning, but it is not always apparent until you remove the head. The rattle sounds like a badly adjusted tappet, and and as Muskrat has said, normally the rattle is absent on a cold engine, but becomes increasingly audible as it warms up. If you lift the head, the guide will appear to be tight because it only becomes loose when the head is warm, although it might just be loose enough to drop out once you have removed the valve spring. This problem is quite common on ally heads, less so on iron heads but it is not unknown for iron heads to suffer in this way too. Also check the left hand plug. Does it look as if oil has been burnt?
John