98-02 accord HELP

my friendc has a 98 accord with the 2.3 4 cylinder vtec engine he has aem cold air intake and dc header nothiung else done to the car today all of the sudden his engine is knocking really bad whenever he gives it gas REVS UP just curious can someone tell me things that could cause this???

well i heard it today its more of a pinging sound like a grip of marbles inside his engine.... wat could cause this someone pleasae help any info is helpfull thanks