Could be a lot of things. First thing I'd do is grab your defrost vent and pull up and pop it out. Then re-install it, it snaps right back in. Mine did that for awhile even after I did the whole interior swap and tightened everything down.
But honestly there is so much there it could be anything.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.