help!!

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i just bought my civic when i test drove it seemed perfect after a couple days past it started making a knocking noise on the top end of the engine, i accelerate it and it sounds like little knocks and ticks when the car is in neutral some times you here a knock other times its quiet.. can any body tell me whats my problem i don't know if its a rod knock or one of my lifters?......its a 1.6 signal cam vtec