a fairly typical V6 engine rebuild, used all new internal parts, heads machined at
shop which has done hundreds of heads for me in the past.

within the first month, did 2 oil changes with new filters(at 50 and 500 miles) and
analyzed oil with great results (no residue, no metallic, just pure oil)

now, customer calls me (it's been about 600 miles driven since assembly) with severe
knocking sound from passenger side valve cover upon startup.

I'm thinking broken valve stem or rocker-arm assembly failed but have to of course see
when I take it apart (have not seen engine yet)

any other ideas?