2007 ES350

I've noticed an interminent buzzing noise on cold starts. Sometimes it's quick and last 2 seconds and other times it last like 4 seconds or so before going away. it sounds very similar to the noise in the video below.

In the video, the dealership told this Ford owner it's a loose heat shield.

i was thinking maybe it's my belt, tensioner, pulley? you don't hear it anymore after cold start.....and it always happens as soon as the car turns on and goes away 2-4 seconds later...