my 2000 ranger has a engine knocking sound, I change the oil and filter yesterday and the sound got louder .What could be the problem?

Is it a constant knock or just on cold starts?

Need more details. Location, rpm range, etc. Could be as simple as something loose or OHV valve train noise or something major like a spun bearing.

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2000 XLT 4WD 4.0L OHV, 5 spd, Alabama Cylinder heads, Delta Cams Valve Train and Pushrods, JBA Headers, KKM Cone Air Cleaner, BBK Throttle body, Dynomax Cat-Back exhaust, Summit Racing drilled front rotors w/ EBC pads, Cobra rear disk brake conversion w/ Hawk pads, 8.8" 4.10 Traction-Lok

I'm having a bit of knocking coming from my OHV too. It seems to come and go as I drive, but definately more noticable upon start up. My engine has a lot of kms on her, I'm thinking it could be some worn lifters. Sound about right?

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91' Supercab 4.0 OHV, 4x4, 5spd, 275'xxxKm
http://www.cardomain.com/ride/3924116/

Most likely.

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2000 XLT 4WD 4.0L OHV, 5 spd, Alabama Cylinder heads, Delta Cams Valve Train and Pushrods, JBA Headers, KKM Cone Air Cleaner, BBK Throttle body, Dynomax Cat-Back exhaust, Summit Racing drilled front rotors w/ EBC pads, Cobra rear disk brake conversion w/ Hawk pads, 8.8" 4.10 Traction-Lok