At least is wasn't the third row seatbelt. When that comes loose and hits the rear window, it sounds like someone hit the window with a hammer! I had this problem with the second and third row middle belts. When I took it in for the first oil change they replaced both seatbelt holders (under warranty) and they haven't come loose since.
Before inserting both clips into their respective slots, pull the belt out all the way, then insert the two metal parts where they go, and then slowly let the belt retract back. This locks the belt in its position and should keep the buckles from hitting the window when it does pop loose.:4: