I would never say the GS is noisy. Maybe there is a problem with your tyres, low pressure or worn.
The squeaks and rattles though are the norm for the GS unfortunalty. Much better in the MKII over the MKI but still there. Some people don't notice or mind but it drives me up the wall. Every air vent, the centre console, the sunroof, the glovebox, the wood trim on the doors all creak if you push them or when the car goes over a bump. The sunroof has been improved on 2000+ cars but even a new car in the showroom will creak as I demonstrated to my dealer today. I also know of several owners, including myself, who get wind noise around the drivers door.
Drive a £14,000 cheaper car, the IS200, and it feels much more solid.