If your picture is of your car, then the lower air dams you have are the "chrome package" ones.
They are probably held on with the same cheap double sided tape that the plain black ones are. check the gap around the entire thing. If there is any "loose" spot, you're gonna have to replace the tape.
The plain ones on my 05 were rattling a bit so I replaced the tape, it's not all that hard.
Just get some 3M double sided foam core tape.
Remove the lower air dams.
(they just pull off, go slow and carefully, there are 4 plastic "clips" that align them)
Clean off the old tape until it is totally gone. Also a good time to give the air dams a good cleaning.
Replace the tape in the places it was before, but don't pull the backing off yet.
When you're done with the dams go back to the car and make sure there is nothing else vibrating/loose in there.
Then clean the contact areas where the tape will hit with rubbing alcohol or something.
Pull the backing off the tape, Line them up press the tape into place.
Voila! rattles hopefully gone.
Hope this helps.