When I rev my engine, I hear a knocking that comes in and out between 2,000-2500 rpms, but it only lasts for half a second as im revving the engine up and I dont hear it when im inside driving. I heard it when I first bought the car w/ 35k miles last year, and now have close to 100k... still doing it. My valves make a lot of racket as well, especially when the engine is cold, but none of wich causes for concern. Aluminum engines can be noisy, especially 4-cylinders. Plus, my car is 8 years old approaching 100k miles. It runs like a champ and is reliable.. thats good enough for me.