To find the exact location:
1. Consult your vehicle's repair manual: This is the most reliable source. It will have diagrams showing the sensor's location.
2. Use an online repair manual: Sites like Chilton or Haynes offer online access to repair manuals. You'll need to select the correct year, make, model, and engine.
3. Search online forums: Search for "[Your Engine Size] 2008 Chevy Silverado knock sensor location" on forums dedicated to Chevy Silverado trucks. You may find pictures or discussions that will help you identify it.
Without knowing the specific engine, I cannot give you a more precise location. Remember to always be cautious when working on your vehicle's engine. Disconnect the negative battery terminal before starting any work.