Hi Scotty, I have been having this noise for years now and had no luck of finding the solution.
It's a 2012 Mazdaspewd3
Rattling noise. Appears after warmed up. Louder when AC is on, engine braking, or at the beginning of revving in neutral (sometimes even neutral idling). Disappears completely when clutch is disengaged.
Transmission fluid had been replaced(75w-80 as the manual asked), clutch and flywheel are also replaced. Noise remains.
Scotty you're my only hope now.