How do you access the gearshift lever ball containg bushing on a 355?

It's from below and quite straightforward. You will want to clean up the entire mechanism. I replaced the ball and the nylon "cradle" it sits in (part no 154741). I think that replacing the ball was overkill - why should it wear. It was also Ferrari expensive if I remember correctly (10 years ago) but 154741 was cheap.

 

I was searching this recently and dug up this thread. Might be helpful.

http://www.ferrarichat.com/forum/348-355/121540-355-shifter-rattle-buzz.html

 

I should have said in my first reply that the first thing to do is try adjustment of the gearbox linkage to ensure that the gear lever is correctly positioned within the gate (the plate against which the lever light be vibrating). If this does not stop the buzzing then there could be wear in the parts 6 and 10. In any case, you have to get under the car!
Adjustment to the linkage is achieved by working on part #36 on page 31 of the manual.