I've noticed over the past week that when I start my car, I've been hearing this "tapping" noise. I hear the noise between 1500 and 2000 rpm while driving. Also, today when I started the car with a cold engine, it revved up as normal for the choke and then nearly cut off, it dropped the rpms down to 500 or less, then revved back up, then back to 500, then did fine, but still had the tapping when between 1500 and 2000. I can get the tapping just by patting the gas at a stop. I've checked all of my fluids and all are fine. no CEL. Any ideas?