On start up there is a rattling noise under the hood. It last about 10 seconds. Then all quiet.

What is your question?

Sounds like plastic rattling. It's pretty loud. My neighbor can hear it in the mornings when I start it up. I can't locate where it could be coming from as I only have about 10 seconds to find it. Once the motor is warm, it will not make the sound again until complete cooldown of the motor.

What makes this problem better or worse?

only on start up of cold motor.

How long has your 2015 Nissan Altima had this problem?

one month. I just got the car