kilgore

well about a month ago i noticed my car ticking and discovered lifter tick, then last week i was driving and the ticking became quiet and about a mile later my service engine light came on. i have not seen any smoke come from the rear but my friend said that he saw some come out. the car also has an exhaust leak, nothing major but its there. my plugs are only about a year old and i have checked my wires. i also checked my o2 sensors and they were fine.

description of the car:
00 trans am
long tube headers to cat back
intake
maf
73000 miles

The ticking became quieter after i ran gum out through the car, so if some carbon built up could it have moved somewhere else? I am basically asking what else can i check.