It's a poltergeist.
Rattle from the outside? Put car on lift and check suspension (sway bar, end links, control arms and bushings, tie rod ends etc). Also, heat shields (exhaust/cat) may rattle, often only while engine is cold. Pebble trapped inside brake caliper or similar may be noisy. Also could be lower engine cover (Maybe a fastener fell off).
Rattle from the inside? Could be anything loose (trim, a wire/cable). Follow the noise.
The list of possibilities is rather endless and approaches the total sum of parts that make up the vehicle!