well bout 2 weeks ago i got my magnaflow Y pipe on and the same day some knocking sound started to come from the bottom of my truck. so i got my fat *** under the bytch and it def. sounded like it was coming from the cat on driver side, now the knocking has become more intense and i cant tell where its coming from.... if was a lifter or rod wouldnt the check engine light come on?

 

No, you wouldn't get a check engine light unless there was a misfire or another fault that the PCM picked up. Could you just be hearing more engine noise that is now amplified due to the aftermarket y-pipe? Or maybe one of the cats is defective and rattling?