OP, it's fine. When engine braking at low RPM, feedback from the engine rattles the disengaged gears.
Same for the sound you hear when it's in neutral. Google "neutral rollover" and you'll feel much better. Welcome to the forum and enjoy your cool car.
edit: Sorry, @
humfrz. My brother and I took the sports cars and ladies out to Point Reyes to see the lighthouse. Fun drive!