Re: Knock under hood but not motor
I would suspect the 'knock sensor' is not working. It detects spark knock or ping, and changes the spark timing to avoid.
Add some premium or high octane fuel to the tank and see if the noise is decreased. Best to add to a tank with minimal fuel in the tank to reduce dilution by the lower octane fuel. You don't have to get a full tank of 93 or 97, just add some to a 'low' tank and see if you get any result.
I had a similar experience where I could make the engine knock almost at will. It eventually quit doing that. At cruise it knocked, accelerate and it quit, take your foot off the gas, and it quit, go back to steady cruise, and it would ping.
You could also feed the engine a drizzle of water or spritz from a Windex bottle with the engine idling. The water may be able to remove carbon deposits as it flashes to steam inside the combustion chamber.
tom
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