Just an update to this thread, my CEL codes have gone away. All I've done is run a few tanks of 93 octane and had to reset the battery for an additional issue. I've since put over 100 miles on the car over the past few days, and the CEL has not come back. Maybe running lower octane gas was causing the P0420 and P0455 codes to come on? The previous owner used nothing but 87 octane the entire life of the car. Maybe that's caused the loud noise I've been dealing with? Not sure, as I inherited that problem...
