bought a 2009 prius yesterday

My 2009 prius starting making a rattling noise when my foot is on the brake or going going low speeds. it makes a noise for about 3 seconds then goes away. When i test drove it i didn't hear any noise like that. I love my car i am going to take it back to the dealer where i bought it. My car is certified so they should fix it. But i would like to have some idea what it is does anyone have any ideas?

I also have another question, I have package I and wanted to upgrade my radio with a toyota JBL can i have someone do that if i have a JBL radio? I like having my ipod hooked up to my aux and presently i don't have that or satellite radio?
thank you