Do you have some idea how fast the air swirls as it passes through the restrictor?
Since air has mass, I'm thinking that if it swirls fast enough, can it form a void, or cavity in the center as it passes through the smallest part of the restrictor, in the same way that centrifugal force causes a void in the water as it swirls down the plug hole.
If this effect is even a remote possibility, then spinning up the air in the airbox before it enters the restrictor could only make this worse, particularly if it spins up fairly fast.
Just a thought.