My dad has the 350 and i noticed that on his as well. i dont like it.. my 430 however does not knock like that.
I don't like the sound. It makes me feel that I am driving a car with diesel engine...
Your GS still has a warranty, have it checked out. To me, injector sounds are more clicky and not a knock. I'm on the road traveling at this time but, if I recall the owner's manual correctly, it states to use a higher octane fuel. We have always used regular with fine results but you may be getting some discount fuel that isn't very good. Just a thought. Try some premium from Exxon, BP or Shell. Again, just a thought. It's worth a try because the dealer will ask what fuel you are using.