Like XScarAudio said it could just be your shifting...I have done this a couple times in my GSR when driving hard and shifting fast. You said it only happens when you drive hard...so this mght be the cause. If not it is probably what Slacker said about being the second gear syncro...if it was the clutch you would hear it at every gear. How are your shifts from 2 - 3 when driving hard? Does it do the same?