From about 2400 to 2800 RPM I am getting a "chatter" from the engine. The noise does not happen when cold and gets progressively worse up until about 20 minutes of operation where it then reaches it's max noise (which still isn't awfully loud, but noticeable.) I have a manual transmission. Nearly each time while driving through this RPM range, in all gears, I will get the noise (once the engine is warmed up.) The chatter may be a couple of ticks or a couple dozen while going through the range. If I hold it at 2600, it will chatter unevenly (meaning that it will give a random pattern of ticks: a tick here, a couple there, 4 seconds of silence, a few more ticks, etc.) I say "ticks" but it is really more of a chattery noise. I would like to address it and was hoping for a professional's opinion. My vehicle is a 2002 Toyota Corolla 1.8L that has the 1ZZFE engine, which is an aluminum engine with a timing chain. My best guesses are valve noise or timing chain noise, but I guess I am here to ask you what you think it could be.