I also think you are just hearing compression leaking by the ring which is normal at "flip over" speeds. The ring might not seal well enough at a "flip over" until it gets some gas/oil to it, so that's why you don't hear it until the engine is primed. The diaphragm is very sensitive to air leaks, etc., and never has enough pressure on it to make this sound so I don't think it's coming from there.

It runs good, so I wouldn't worry about it!

AV8TOR