It is because the boards are not properly secured. Has nothing to do with the depth of the box compared to the mounting depth of the driver. You could concievably have a 1/4" of clearance between the magnet and the rear of the box if necessary. The reason it is moving in and out closer to the sub, is because of the movement of the sub itself........ your vent doesn't move.//content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif The board's fluctuation is because of the sub's movement, not the air pressure being generated. Pull the sub out, glue or screw the board that is moving, put the sub back and try again.........