Hi folks. 2001 Silverado LT Ext Cab 4.8L 236K miles all original. Been getting P0300 multiple random misfires. Happens after highway driving about 45-50 minutes and appears when I decelerate like down a hill. (almost like truck breaking action), MIL flashes, code set. Looked at possibilities; Cat. Conv., intake leaks. Believe all ignition good, fuel pump replaced so leaning towards CC and intake. I also get like creaking/ knocking sounds from exhaust (can't tell, maybe Cat. Conv.) , after parked and cooling, occasional once when I first start engine. Any thoughts on how to check Cat Conv., other isolation, anyone seen this before.Thanks.