if its while your turning....cv axle or boot.
what i suggest is turn ya music all the way down and roll ya windows down, try and hear it again while making turns, if its only while turns, try to isolate which side its comming from.
More then like, if you make hard turns and turn the steering wheel as hard and possible, it make have busted the boot.....if so, you can get a cv boot repair kit at Autozone, if its a busted axle, you gotta swap the whole thing out.

also, is it a clicking or your defintly sure its rattling, get back to us man.