Crunchy rattling sound only when idle and starting?

I'm getting a strange rattling noise that is louder from inside the car than outside. It can only be described as some sort of metallic crunching or rattling, even close to bubbling. My friends heard it and said it sounds like some parts might be rubbing against each other. I was able to determine that the sound was coming from somewhere behind my back rear tire, but when inside the car it’s so loud that it seems like it’s coming from under the hood. I’ve had trouble finding any info on it, or even a video with the same noise. This video was taken right after starting the car, but it can also happen randomly when I'm idling at a stoplight or drive thru, and it usually lasts from a couple seconds and sometimes a minute or two. Also, I don’t have any sort of codes or other signs of any serious issue.