one of the spark plugs has gas fouled and is hitting and missing causing a gas popping in the exhaust if at the time its doing it you will notice a hesitation or stumble and than clear up - finding it is the real thing so i would swap out the plugs with Bosch plat 4 iridium about 6 /7 bucks each they dont need to be gapped and they will let it run better than it has its a win win - jz

i would also put a can of sea foam in the fuel tank you might have some delivery issues and that might clear it up -