Confusing noise..

Well, i've searched above and beyond to see if anyone has a simular problem. I've found close but it's no cigar. Anyway..

1990 240sx 94k miles :-) bought it at 1700

At 2k-3k rpm i hear a rattle.. Simple as that. I'm guessing from what i read all around is that it's my timing chain? I also read that it might be my cat. converter's firewall, removed that, nothing still noisey. I was thinking it might be my exhuast but it's mainly comming from within the engine.

also, how much would it cost me to go to z32 breaks from stock?

ttt

I know what your noise is because i had a very hard time trying to find mine!!!! the wrist pin in one of your pistons has to much clearance around it and is causing a knock much the same as a spark knock..
drive easy cause it's only a matter of time. mine made it to 315km's before it let go.. rebuild or swap!! hope this helps