Which came before, the sound or the dream?

[Ajax3]

i had dreams where a noise has woken me up from dreaming. Whats curious tho is that the dream actually lead up to the sound. I wonder what came 1st, the dream or the sound. If the dream came first then how did my mind conceive of a situation to fit it in? One recent dream had a long narrative and ended w/ a creature/monster making a "growl" at me. At that moment I woke up and realized it was from a car engine starting up just then. My brother had a dream of something falling and crashing into the ground and it sound like pencils. He woke and discovered he had knocked over his box of colored pencils, which were still rolling along the floor. How does this happen? If the sound came first then how fast does a dream form? Split second dreaming? Any thoughts?