I had the same thing happen to me.
I took my honda in for CV joint replacement and a week later they were rattling again. He said they replaced them and again I had to go back.. Im not sure really what they were doing, but I decided to go elsewhere and have new CV joints put on and not some rebuilt crap the other shop used. To my happiness, sure enough no problems for 2 years now..
Maybe get it checked by someone else??