is it more of a thud, or a double click and how long has been since u changed you oil, check the oil level. does it get louder with rpms, and is louder when its cold and then gets some what quiter when the engine gets warm
Is the 'check engine' light on? If it's not a mechanical issue like valve adjustment or a rod/bearing knock, as the other response addresses , it may be normal 'piston slap' which would get quieter as it warms up. That motor does not have a distributor , so if it is actually pre-ignition or 'spark knock' you may have a faulty knock sensor or a vacuum leak somewhere , causing a 'lean' condition. Need to know more specifics on conditions to properly answer. May be best to have a shop scan 'live data' and check fuel trim for starters. Also TPS for proper values.