One problem related to engine knocking noise has been reported for the 2004 Chevrolet Trailblazer. The most recently reported issues are listed below.
The contact owns a 2004 Chevrolet Trailblazer. While parked with the engine running, the contact heard a loud knocking noise coming from the engine. The vehicle was taken to the dealer for diagnostics. The technician advised that the knocking noise was normal for the type of engine in the vehicle. The vehicle was then taken to an independent mechanic who advised the contact that the pistons in the engine were down sized, causing the noise. The vehicle was not repaired. The approximate failure mileage was 53,000.