I'm not sure if this applies to the Direct Injection system in the IS, but many GDI systems eliminate the throttle assembly, thereby eliminating an obstruction in the intake and reducing pumping losses. Engine speed is instead controlled strictly by the amount of fuel injected into the cylinder (much like a diesel).
As has been mentioned before, GDI technology is similar to that used by diesels, and the ticking noise you hear even sounds a little like that of a diesel.