A rattle you can hear from outside, or just inside?
A good thing to check is the sway bar end links. They go bad often and give a loose internal rattling sound. Without knowing they're bad you would never know by looking at them.
They are cheap and available at autoparts stores so that's one area to check.
But also look at your pads & rotors, all the parts there? Hows all the bolts? Any looseness in the front end elsewhere? Car up on jack, how does the front left feel?