Since the "rattling" sound goes away once the engine warms up this means the mechanism for the choke is most likely the culprit. All chokes need to be adjusted, even your type. My guess since it also runs rough cold the choke may be set to hard so that the linkage is pushing them to much in the closed position. With the motor cold & off loosen the three screws on the choke assembly and turn the cap until the butterflys just start to open then back it off a tad. (see link below)

Just a question... do you hear the rattling when the breeather is off also? Could it be that's making the rattling? Does your new carb have the same radius opening as your old one? If it's smaller than you might have a gap.




Setting Choke: The choke is adjusted, by loosening the three screws on its ôring,ö then
rotating the choke clockwise for less choke, and counter-clockwise for more choke.
Note: The Fast Idle Speed screw is located behind the choke itself, and can be adjusted
to increase or decrease the fast idle rpm.

http://www.cjclub.co.il/files/Weber3236adjust.pdf