I had the same prob when I first got my 15" XXX. It was port noise I believe. Box was real weird, I sealed it and tried it out of the box in the air, no noise //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif It's your box man.
Edit: Maybe not after reading the last post... but still try to run it free air and u can find out whether it is the sub or not 4 sure.