Bill,
You say you've eliminated the rocker cover, how about the rocker box? My guess, and I suppose the most popular answer, will be that it's leaking between rocker box and head. Mine does it (birds do it, bees do it...) This is pretty common due to the head steady tranferring vibration to that joint. Due respect to Andrew, but I think the spark plug possibility is more obscure. If it turns out to be the plug, I benefit by learning somethiing new.
Richard L.