Maybe RainX on the window. Makes the windows very smooth. If you need to get to the gears, the bottom part of the outside door comes off with only a few screws. Very easy access.
I doubt that the glass is causing the chatter. If you feel comfortable doing so, a look at the inner workings of the door with the panel off could tell you what is doing it. Usually chatter is caused by the regulator linkage binding from a lack of lubrication. Of course, Rain-X on the glass is a cheap experiment.
I would pull the outside door panel and have a look at the window regulator. In particular the 3 rubber dampened screws that hold the window motor to the middle of the regulator. They have a tendency for the screws to pull out of the rubber and thus allow the movement of the motor from the regulator. This thread will give you a better understanding of what I'm talking about. http://www.ferrarichat.com/forum/showthread.php?p=135816990 It is for a 348, but it's still the same deal for the 355. They use the same window regulators.
Chatter from what? Does it sound like the rubber or felts rubbing against the glass? If so, some careful spraying of silicone spray on the window felts (tracks) and rubber can quiet them down. If it is more mechanical in nature, then the door panel needs to come off so you can inspect the window regulator. As mentioned, the door panel is not difficult too remove, just remove all the screws on the perimeter, and the door latch philips screw. The center portion needs to slide off. http://www.the355.com/mambo/showpage.php?id=124&page=286 #19 comes off by sliding forward or backward (after removing the screws) You also need to remove the triangle piece at the forward top edge of the door.
Good post Ernie! The other welds that can fail are where those two spot welds are for that "bridge" piece. I recall someone having rattles from that breaking too. Their fix was to drill and install a small bolt and not there to tighten them together again.
