[thstone]

I have the same problem on my Miata. I believe that this is most likely an early sign that the clutch is on its way out. The clutch on my Miata doesn't slip in gear, it just chatters at engagement from a stop. My workaround has been to be smoother with the pedal engagement and give it a little bit more throttle to get a smooth start, but I expect that the clutch will start to slip in the future and ultimately need replacement.