It may not be a vacuum leak. Most vacuum leaks get louder as you accelerate and disappear as the load on the engine decreases. They normally do not show up a certain speed. I am not rule ling it out, it just seems unlikely with the information given. Will love to here what you finally out the cause. Yes I do know how much Ford loves to use vacuum on their engines.
Not trying to start anything, just my opinion.
Very new to the form, have learned much reading the threads, so thanks.
2014 Fleetwood Storm28F, 2008 Prius on TD.