I'm having troubles with my HVAC. Sometimes (especially at highway speeds) I get no air at all to come out of any vents, hot or cold. However, I can hear the fan motor running. Most of the time it works. Is it possible that it sucked in leaves and that's choking off the air or is the motor just going out?

 

Air should come out of the defrost and floor vents as default. Low vacuum to the vent controls can cause air not to come out of the dash vents. A leaky hose, check valve or vacuum tank can cause that.