ive had this problem for a few months now and just deal with it. ok when my car sits for awhile and the car gets completely cold, i start it and for about the first ten-twenty seconds cars fine with normal cold idle. then i start to get like a small litte rattling noise(not sure how to describe it, you can tell what ever is making it is spinning or moving by the variation in the sounds). it last maybe 30 seconds getting quiter as it warms up. the noise comes from the head area between the dist., and oil cap,possibly coming from back by injectors and echoing over. ive heard maybe the ditributor bearing???..... car has 90k on it. this may sound stupid but i had the timing belt tightened when i brought it it for the noise one time and im pretty sure it went away for awhile.....then came back. not 100% sure at the time it was doing it periodically.i have already put a brand new itr bottom in so im hoping its not expensive.

 

catalytic converter? buy largest clamps at home depot, place around the cat and see if that takes care of it.. if it sounds like metal rattling, then its exhaust heatshields

 

i dont think its that because i can put my ear to the junction of the valve cover and the dist. and hear it. i doubt is resonating all the way up fromthere.....