Noise in my 340

Hey guys. I've finally got things sorted out with my 340 as far as getting it running. It runs great but I'm getting a noise. I can't seem to tell if it's mechanical or what. My buddy describes it as a slap. There is a small exhaust leak under the car, but i dont think thats the cause. What do you guys think? My grand dad didn't think it was mechanical. Just for info it has stock rocker arms. Below is a link to the video on YouTube. Thanks fellas.

http://m.youtube.com/#/watch?v=14QNfrnY6Ng