Car shakes and rattles when accelerating?

I bought a brand new 2013 ford focus se last April. In the last month or so it has started rattling and shaking when I first accelerate. After a moment it stops making any noise or rattling completely. The oil changes are up to date. It doesn't stay in a garage, though I don't know if that has anything to do with the problem, just throwing that out there. There are only 12,000 miles on the car. Any thoughts?

If it also seems a little low on power you could be running on 3 cylinders. I love Fords (had many Ford Rangers) but the one Focus I had sucked an intake valve seat (started shaking, rattling, low on power, burning oil). Ford should have recalled them but didn't. Shame on them! Under warranty? Get it in quick!

Was the car crashed before? It might something with the tires or maybe a bent axle from a previous crash.
Did you make sure all your tires are inflated to the proper PSI? And don't have any weird bulges?
you should probably take it to a mechanic, rattling usually means something is loose.
Loose usually turns into something coming apart, not something you want happening at highway speeds…

You may have a turtle stuck in your tire. Turtles getting lodged in tires causes cars to shake. Make sure you put turtle repellant on all of your tires in the future. For now, you have to wait until the turtle gets bored and leaves.

This is what the warranty is for.