Where is the knock sensor located on a 04 Chevy silverado 5.3 liter?

The knock sensor on a 2004 Chevy Silverado 5.3L is located on the passenger side of the engine block, near the front. It's usually mounted fairly low, and often near or below the oil filter. You'll likely need to get under the truck to see it clearly. It's a small, threaded sensor.

Important Note: Due to slight variations between engine configurations, it's always best to consult a repair manual specific to your vehicle's year, make, model, and engine code (found on a sticker under the hood) for the exact location. A picture in the repair manual would be invaluable.