Q: Loud Knock

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Knocking noise under car after start up, didn't start until put I drive and slightly driven up the hill in my driveway then put in reverse when I notice and came back down. Now at idle loud knocking noise and car barely stays on. After pressing on the gas, the knock dies down but starts back up at idle. Any clues?

My car has 110000 miles.
My car has an automatic transmission.

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