So my car has been rattling from the exhaust for a while now, almost since i got it, I finally go today to go check it out and what do i see but the top heat shield on my cat is off. So i pulled the thing out. Is this something that should be fixed immediately? Also what there seemed to be some grease/oil on top of what i think was my cat (i'm a newbie), should there be grease anywhere back there? Thanks a lot

John