Every transaxle equipped Alfetta, GTV6, and even the Milano (but not much in comparison) we've owned has had this rattle, and when I first commented on it to Carlo, he said that Alfa said it was the transaxle internal gear train making the noise due to neutral roll-over. inherent in the design of the transaxle drivetrain.
He said live with it.
89 Milano (wife's daily driver since 1989, Shankle Sport)
91 164S (my daily driver since 1994)
94 164LS (~Q) (trip Alfa since 2000)
72 Morgan 27 (water time since 1976)
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6