(P5-automatic) i'm not really sure if it had it since new, but i can always hear this clapping noise on a cold engine ('bout 3-5 minutes until it warms up)... it's also chocked and it changes gears around 3000rpm. I took it back and they said it scored ok on the computer and it's nothing wrong with it. I don't find this normal though... oh, and i also realized recently it started having a hissing noise when accelerating... it's only 3 month old and i didn't pushed it "way" too much (i hope)...

 

I have an '01 ES although mine is manual transmission, I think I hear the noise you experience. When it's cold and I don't let my engine warm up long enough i hear a slight knocking. I"M NOT SURE SPECIFICALLY what is doing it, BUT when it's below 60 degrees, I let my engine warm up until I see the needle on the temp guage start to move, and even then I don't take it past 2000rpms till the needle is in the middle of the temp guage.