Join Date: Feb 2007
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Yeh this is quite common when fitting roof bars and boxes - it's known as the venturi affect when air is forced through or over an opening and speed causing a vacuum. (similar to blowing accross the neck of a bottle) When you open the window it allows air into the low pressure area and lets the pressure equalise. Can you raise or lower the box as this might help - the other way is to have a air dam fitted on front to reduce the speed of air traveliing between the box and the roof - that's why you are warned to remove the roof rack or box when not needed as it also decrease the MPG substantially. Maybe the box is not designed to be used at the speeds over 50 MPH or so.
Currently in madnoel10's garage:
Honda Civic 1.4l