exhaust help

i just got my new apexi n1 exhaust on the other day(by the way its great and i love it nice tone and gains for anyone looking for exhaust) and when i am accelerating and down shifting there is a loud rattle that sounds like it is comming from directly beneth me. before i put this on i had a problem with my shifter hitting the pipe but it would normally stop when i put pressure on it. when i put pressure on it now nothing happens. what might this problem be.

Do you have a short shifter? If so it might be touching the guard around your cat.