Yep. Can add that the noise is a lot more prevalent in cars with light flywheels. Here's a thread from Miata.net where they talk about the rattle associated with light flywheels, with one person describing it as I usually do, sounding like marbles rolling around inside the transmission. I think the consensus is that it's gear lash, and the gears in the trans are rocking back and forth between power strokes, so the lighter the flywheel and the slower the idle speed and the lopier the cam, the more likely you are to hear it.
https://forum.miata.net/vb/showthread.php?t=732227&highlight=flywheel+rattle