dynomax cat + side pipe problem

[redbird8628]

this past tuesday, i put a side pipe on my 2.8, its just a dynomax 2.5 " cat and a pipe that exits just before the wheel well. It sound great,very quiet compared to the open y pipe it ran for a few days and little to no rasp. Just today, i started it up and it was loud as hell, sounded almost like i had the open y pipe, there are no leaks. Its loud at low rpms under a load and raspy as all hell aproaching 4k, could i have blown the cat out already? what do you recommend to quiet it down i little more, i was thinking a exhaust insert, any ideas? may it have anything to do with me switching to 89 octane from 93, i did on friday afternoon, i can hear the egr working its a** off under even a slight load now.