Knock sound

[Brandon M.]

Ever since i put an oil change in my 1994 explorer (recent oil change) none before that i got a knock on the passenger side sounds like a valve but it maybe a lifter cant really hear it much unless i put stress (a load) on the engine then it get's to where i can hear it easy any ideas on whats causing it? it has had a slight decrease in performance not sure if that has to do with it or not and no check engine lights not on and ive tryed different weights of oil i know i need to change the oil sending unit or pressure gauge just havent had the time to tear into it and replace it. i heard somewhere it could be ignition? im going to test that later to see but if not that then what else could it be? and what is a good transmission to put in a XLT explorer limited and EB (as of 02/20/2010) it has been idling really rough for the first minute of it running cold and when i rev it up a little bit it dies on me and then starts back up and runs fine till it gets cold again