Hi Martin, the latest diagnosis by Lexus Notts is shock absorbers. I'm actually dropping it off this evening. The GS430 was very fast, although I was a bit disappointed with the ride. I thought that it would be much better than the Sport as it was an SE, but to be quite honest it wasn't that much different.
I don't think that my knocking is brake related. If I drive along with my foot on the brake, the noise is still the same, and there is also a grinding/squeeking noise when turning sharply at low speeds. I think that this is probably the shock absorber top mount. We'll see anyway... :D