That flapper is rubber and is does not make any loud noises, I have the internal flappers from stainless marine and they chatter, so i would not think it was the flappers.

just 02.

a buddy's rebuilt 502 was making this noise after a couple of weekends use. The builder was supplied with a set of recon heads with valves installed, turned out that the valve guides were oval, not the builders fault as my buddy didn't want to pay to service the heads. The valve ended up dropping, puched hole in head water jacket, hydraulic lock up bent rod etc. Moral of this story, let the builder check out supplied parts in engine build.

Mark

BTW $s saved 175 dollars lost $3500

Broken valve spring? Have to look close sometimes to spot it.