I have an 05 Envoy and when you have the heat on and set to come out the floor vent only, it comes out the defrost vent also. Also doesn't seem to be moving as much air as it should. The fan runs fine but seems like there is a blockage. On my F-250 when I have the vent issue, it is a vacuum leak. Anyone have any ideas on what I should look for to fix these issues?