It may not be bad gas, but it could be the wrong octane rating for that engine. Check your owner's manual and see what octane is recommended for that engine. If you're running 87 octane and it requires 91 or higher octane, then you need to be running the higher octane fuel. By running lower octane fuel in an engine that requires higher octane, your getting pre-ignition, the knocking sound you hear. What's happening is that as the piston moves up on the compression stroke, the gas/air mixture is compressing and as the pressure goes up, so does the temperature of that mixture. It's possible that temperature can get so hot that the gas/air mixture ignites from the high temperature and not from the spark plug. Higher octane fuel will help this since higher octane fuel is harder to ignite. I say try running some 92 or higher octane fuel in your car and see if you still hear the knocking.
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