Dakota rattle noises

Ok, when my dad bought this 98 dakota sport club cab, it had a wierd rattle at about 2,000 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 2,000 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 CAT's?
Thanks...... Trisha southern california
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