Oh, you don't have a gurggle sound you have a HVAC sound. Is your AC hose cold to the touch outside while the other AC hose is hot? A vacuum leak should create a malf in the controls so that is out. Did you mess up the connection of the hoses thru the firewall. It is easy to dislodge one of the seals or the spacer between.
Watch all of his videos, great tips on repairs to a heater on a F150.
After watching them you can remove the connector assemblies and recheck the seals for damage or dislocation inside the thing maybe even clogged/bent. When you are reved up at highway speeds you are moving a lot of water 5 gallons in no time. It wouldn't take much of a restriction to make a noise.
Also check out his blend door tips for the new repair to them. Slick bushing trick to reinforce the area wher it breaks and make it fit into the actuator.