Like Breck says... the rattle is possibly a heat shield. The most common is the one on the exhaust manifold. Then the one at the Y-pipe. Or both at the same time occasionally.

Could the rattle be valve noise? Kind of a clicking noise? If so, it might be either that or a bad exhaust manifold gasket. Cracked exhaust downpipe. There are a few places to look for a clicking noise.

Feel the back of the engine, especially the back of the head, to see if it has oil on it. That's the most likely place for oil to be dripping down the back... if there is oil back there, check to see where it is coming from.

And clean the flame trap. That should be a monthly mailbot message. Clean your flame trap. I have seen cars with them totally plugged, and that will definitely eventually cause a burning smell!