The RPM doesn't matter. I could shift at 1800 or 3000 RPM, sometimes you'll hear CHUNK (sometimes soft, sometimes louder) and most times you won't. I am careful with clutch engagement, I don't dump the clutch. It does it only when shifting between 1st and 2nd gear. I bought the car used recently and don't know the car's history.
My 2001 Boxster makes a chunk noise when shifting between 1st and 2nd. on 2001 Porsche Boxster
This week's top 10
Recent Porsche Boxster Questions