Strange Engine Sound

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Mazda Protege5
I've noticed that when my P5 is cold, there is a muted "whap-whap-whap" sound when you rev the engine. Once it's warmed up, it stops. It doesn't go away gradually, it just stops. It sounds like it's coming from the top portion of the engine somewhere. I'm thinking it's something emissions related, but have no idea what. Anybody know what's causing it?
 
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