I just taped one of the stickers in the bottom left of my rear window. I travel in the the car pool lane every day and have been passed by the Highway Patrol on many occasions. I've never once been pulled over and told to slap them on my bumpers.
I also had the rattle last week. Pounded on the console a bit and it hasn't come back.
It could be that one of the clips is loose or wasn't set correctly.
After ordering my Leaf, I realized that I needed another car ASAP for my son. Went to Mossy in Poway, where I ordered my Leaf, and asked if they had a lower priced used car. They had just taken a trade and I bought it for $6,500. I paid 5K of that on my Visa. No problems.