Hmmmmmmmm, not sure. Something that seems kind of solid hitting something that sounds thin and metallic.
Tell us more. Which year, make, and model? Did it just start? Did you recently have it serviced? Does it need service? Did you have any work done on the car? Modifications made to it? Anything loose in the rear trunk? Do you hear it when going fast? Does the rhythm of the noise speed up or slow down depending on your speed?