I got that too, virtually every time after the car has been driven for a while and is warmed up. Also, clutch feels less... tactile and tight on engagement. My best guess is something in the transmission loosens and/or softens up, so you get this resonance both while idling in neutral and during that last little bit when the clutch is being let out; both emit same kind of sound.
Besides less satisfying clutch engagement and annoyance, this doesn't seem to affect the driving dynamics of the car. I am, however, super interested if anyone manages to get a mechanic to diagnose it to the end.
EDIT: The car is bone stock.
EDIT #2: Re: more clutch slippage, if I do it slow, it makes it worse; if I'm aggressive and quick at the same time, I am able to eliminate the rattle about 50% of the time, but that's not how I would necessarily want to drive the car.