While the Model 3 warranty doesn’t officially cover rattles, the service center will often fix them as a courtesy to customers…if they can find them. Rattles are often hard to reproduce and owners are left frustrated. I find it’s usually easier to fix them on my own.
I had an intermittent rattle coming from the armrest area of my driver’s door panel. After taking the door panel off, I found a connector in that area that was just a bit loose, so I used felt tape to secure it and its wire. Problem solved!