I recently went through the same problem. I went through and pulled my valve covers and checked the rockers and pushrods.. nothing, thought the problem was fixed.. warmed up then the sound came back. Pulled the valve covers again to check if the rockers were all getting oil, fired it up and watched for oil in each rocker.. all was well. when i was finished putting it back together again i started it up and noticed one of my spark plug wires was really close my exhaust manifold, and when i grabbed it and began to move it I felt hot air and heard the noise... My spark plug had broken around the porcelain in the middle so it was blowing by the spark plug making that perfect valve slap noise haha!
If this isn't your problem try checking your rockers for oil. pop the valve covers and start it up.. I have the 3.0l and it didn't make a mess at all, but im not saying by any means you will have the same result haha. Just glance at each one, sometimes it makes it a little easier if you bump up the idle a little bit if it is hard to see the oil.