Dakota rattle noises

From : butterflytatt2s

Q: ok when my dad bought this 98 dakota sport club cab it had a wierd rattle at about 2000 rpms. the noise got a lot worse well we thought it was the same noise and it started to rattle all the friggin time it was horrible. after a few months yesterday the check engine light came on. so my dad took our truck to a shop who replaced the cat convertor for $80 because the mech. said it was a knock off brand and it wont pass smog compared to the $250 cat that will pass smog and the loud rattling noise is gone! but......... that same rattling noise that was there when we got it still rattles around 2000 rpms. it isnt that bad but i am guessing it might be the exaust manifold. oh and about that check engine light after the mech. replaced the cat he told my dad that the cat wasnt the cause of the light. he said it was some code p0422 and he just suggested putting the gas cap on tighter. does anyone else have that little bugging rattling noise and is it an easy fix and is there really such a thing as a cheap knock off brand of cats thanks...... trisha southern california .