Now that I paid attention, I do hear my engine knocking between 1800-2,000 rpms while driving, and it does sound discomforting. I heard it while driving home from work, so I parked my car in the driveway, lifted the hood and reved the engine via throttle cable, sure enough, at 1800-2,000 rpms the knocking came in... it sounds like a grinding/knocking, and I can actually feel the engine vibrate a bit more when it comes in. From inside the car, holding the engine at a steady 2,000 rpms, it sounds like Im grinding coffee. If I rev higher, it tends to fade out. I dont know what I could have done to cause this sound to appear. The engine has close to 100,000 miles, mostly highway, and all run on Valvoline synthetic oil.