My 2002 Corolla CE gradually developed a slight and intermittent knock/rattle sound almost only at high speeds (60+) and upon increased load (e.g. even slightly uphill). Very rarely it happened lately at lower speeds when I had to push the car hard to merge uphill. The noise is faint and it sounds like when someone is churning a chain against a metal surface. It comes most noticeably when I push the gas pedal down just slightly when going uphill (narrow throttle). It almost goes away at wider throttles and disappears when I reach the top of the "hill". I keep the car in a really good shape, just did a 100 K service on it myself. I have 1.5 year old iridium plugs in it and run oil and injector cleaner through the engine several times a year. I did not see a significant carbon buildup last time I checked, but it is not easy to see though the plug holes. I read a lot about engine knock and carbon buildup and various removal techniques (e.g. SeaFoaming your engine). I don't agree with those who try to make the case that a knock is "normal" for any good car. It think it shouldn't be. I also read about possible pulley and belt issues that may cause similar noises, but usually at all speeds. Most of my driving is on the highway and I am quite gentle with the car. I just wanted to pick your brain before I start taking things apart. Any input is much appreciated.