searched and found nothing related to my issue and that surprised me.
i've had this NC for 8 months and knew about the rattle at that time. But it was a great deal with 50k miles (still under warranty) and with some good service history.
quite a long story- sorry

if the engine soaks outside and it gets down to about 30f the start up has no noise,,, if it sets in the garage at about 60 f it has the start up rattle,,
occasionally i hear a sharp metallic pop when it fires and that is very concerning
tried wix, mazda filter - no diff
0-20 and 5-20 (5-20 is less noisy)
if i hold down the gas and crank for about 5 sec there is no start up noise.

it sound like VVT actuator but i'm not aware of the nc issue.
after it starts its noise free.
the dealer kept it over night but left it outside and said they heard no noisy even though i told them to leave it inside,,, it looks like i have to tell them the issue!!!

anyone seen this issue? thanks for your input.
very nice forum,,
david