The attle is from the heat shielding on the rear manifold exhaust near the firewall. If you do not know where that is, take it to a mechanic and tell him you think that is where the rattle is coming from. Most loud rattles come from the heat shielding that surrounds sections of the exhaust.
Now if you've hit something recently, ran over something, had some work done. etc. etc. who knows. A two sentence blurb on the internet is not really a scientific diagnosis. I am just going off of personal experience and others here that have had similar issues.
Is it safe to drive, yes most likely, unless you notice issues while driving.
Every man made machine will eventually fail, the real question is how good is the machine while it works!