Engine makes a "CHUFF" sound when I turn it off
I just rebuilt my alternator and while I was already greasy I dropped in a new set of plugs. I used Bosch +4 iR Fusion (Platinum/Iridium surface gap hybrid). When I got everything back together and went for a test drive it was fine. When I shut off the engine it made a "chuff" sound through the airbox (pretty sure) like a valve releasing compression from a cylinder. It's never done this before but now it does it every time. Any ideas?
It's a 2003 330CiC Sport.