[MinionM3]

I have the exact same thing - possibly always been there but just started to notice it about 3 weeks ago. I am 6MT as well.

Metallic rattle, sounds like it's coming from directly under the car IMO, it is at very low RPM, like first gear and gently pulling away, or a lazy 1-2, and not giving hard throttle, hear it in 2nd as well.

It happens for a second and is gone.

Seeing the heat shield comments makes me believe that's what it is - I definitely do not think it's coming from under the hood or at the very back of the car - really sounds to me like it's right under me in the middle of the car.