Popping in the exhaust when free revving the engine is a symptom of a lean mixture. Only being a problem when the engine is warmed up means it's not injector or filter related, as mentioned. When cold, it's in open loop, and rich. That's normal. I am suspicious of the O2 sensors, but you really should run a KOEO and KOER self test and see what codes come up.
Take care,
-Chris
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