Airflow and gas pedal

OK.....the "chirping from steering wheel" thread isn't going anywhere......
How about this:
When I step on the gas, the AC airflow changes from the vents on the front of dash to the defrost vents at the windshield. When I take my foot off the gas, it goes back. It is annoying in the Arizona heat.
Does anyone have an idea why this is happening?
I'm thinking something with vacuum?
thanks for your replies.

First, you need to check your vacuum supply line under the hood going from the manifold to the vacuum canister mounted on the firewall. It could also be a leaking canister! The engine pulls higher vacuum at idle, therefore, it would be logical that the system would operate at idle and not under acceleration. (Lower vacuum)

Yes there was a hose at the "T" over the manifold that was not connected.
The airflow is more even from both locations now on acceleration.
thanks.