If it's still under warranty just take it to a dealership when you get a chance. They will find the noise and fix it all for free!
Mine does exactly the same thing. I figure: If the dash doesn't rattle, then the car is not really a Nissan. LOLI just bought a 2011 Sedan - and the entire dash seems to squeak especially in the cold. Other times it sounds like there is something in the dash. But it's never the same thing every day - always different. And then other times I hear nothing. frustrating. I hit the dash and it doesn't help.
I think that spray is called "Great Stuff".Their's this foam in a spray can at Home Depo. It's used in construction for homes that have gaps in walls, doors and windows to fill and insulate. It hardens after it dries great for stopping a rattle. Very messy when you apply it though, but it works great! Just make sure you cover up the floor before using it.