Heard the same noise again today.... I have no idea what it is... And having the car in neutral makes me question it even more...

Had this issue w/ my cooper S. turned out being the flywheel

https://www.google.com/search?q=flyw...&client=safari

this is exactly what i am hearing. i thought it was my front-pipe installed.
I am still confusing whether this is air passing sound but reading this thread makes me that sound is very similar to mine. i also hear this sound put into neutral when slowing down.

losing my marbles
 

My FR-S with manual transmission (3K miles) has been making the same "loose marbles" sound intermittently for the last few weeks when decelerating in gear (for sure in 4th, and maybe other gears). It lasts for a second or two and only when the car's fully warmed up. The sound reminded me of valvetrain clatter on a car I once had before the oil got circulationg but I can't locate it--the other posts indicate transmission or clutch. Sure would like to know if it's serious or just another sound I need to get used to (like the crickets).

I have driven this car 1720 miles straight without letting it cool 4 times now, and the gears still sound like this lol...They haven't gotten worse, although, once heat soak sets in, the gears are audible while cruising.

There is a rattle from the shifter area that the dealer says was something under the hood cause it takes alot of driving to replicate and I won't let them drive my car for 6 hours to get it to do it, even though there is a TSB out for it.

There is the clutch rattle when depressing in low speeds with engine breaking in progress.

There is the loud gears that sound all metallic as hell.

And there is a flywheel rattle that I hear on harder acceleration.