97 cvlx gas tank not venting

My 97 cvlx has a problem as the car gets to operating temp,there is vaccum pressure build up at the filler pipe,which then makes a popping sound until the pressure builds up again,I did replace the fuel cap(oem)this did noy help.also check engine light did not come on,nor has there been a change in the way the car runs.any thoughts would be helpful. Thanks.............David