Hey Tony, my car did that too last time, but only once. Mine is more like a white smoke with dust particles coming out of the middle vents. I put it on max for a few seconds and it went away and never come back. I don't know where it came from though. :ugh3:
Pretty sure I had a car once that blew white flakes out the vents... When it was snowing. Guess there wasn't a cabin air filter on this old beater? Haha. Seriously though, I do remember a car that would blow snow from outside thru the vents!