Dennis,
It's not vacuum, it's an electronic actuator. Early models used a vacuum transducer that was located behind the drivers side headlight. When they put the turbo resonator unit in the 2004 models, everything went electronic. It's feasible that the actuator is failing since there is a rubber "O" ring on it that gets hard but I would think that the tech that looked at it today would have thought of that. Also, I seem to remember that a code would be set if the actuator arm was not moving properly.
If he had a turbo problem, a code would have been set and the ECM would put him in limp in mode.
Ken called me last night, and from what he told me I would look for a broken baffle in the muffler. He no longer hears a "Bang" that only happened once, and was probably the baffle breaking. The hum he hears I suspect is the baffle vibrating as the exhaust pressure builds. Once it is at it's peak, the vibration backs off.
He has full power, no check engine light, no trans issues, just the noise. Given that the muffler is the original with close to 450k on it, That's the first place I would look.
I told Ken to just let them listen and see what they say. If they come up with something else, I told him to have them tap on the muffler with a hammer. If the muffler rattles, BINGO.