Help! Weird noise!

[BRNDNSLVRA]

Somebody please help!

Whenever I let off or brake (mainly in the morning when the car has sat), theres this weird rattle like sound coming from the back. I can't tell what it is. It doesnt effect how the car is driving but something isn't right.

You can hear it in the beginning of the video. After the second small pull when I let off, its louder and more apparent. Turn up your volume. I edited the audio to play the high end frequencies more than the lows.

Im 100% sure it's not rocks in the head shield. The noise lessens the more I drive. I've already checked the heat shields. It sounds like sand almost.