What would cause a knocking sound in the engine when you rev it and oil is at right level from a 2004 Mazda 6?

A knocking sound in the engine of a 2004 Mazda 6 when revving could be caused by several factors, including worn or damaged engine components such as bearings, pistons, or valves. It may also indicate issues with the ignition timing or fuel mixture. Additionally, a buildup of carbon deposits or a failing lifter could contribute to the knocking noise. It's advisable to have the vehicle inspected by a qualified mechanic to diagnose the exact cause.