why does my truck blow air from the vents when blower is turned off?

Thanks 4 the reply. you were part of the way right .I trouble shot for hours and after checking all the vacuum lines under the dash.For all the vacuum bladders come to find out that good old Gm. Has a vacuum line that runs your four wheel drive and your heat vacuum also.After tracing the line Come to find out this vacuum line runs to a little black box.Located in the drivers fender by the master cylinder. needless to say my hose was disconnected.so the selector door was stuck on dash vent all the time. Thanks again Dave