I would be more suspicious of a vacuum leak here, as opposed to an air intake duct leak; that woulld probably be accompanied by hesitation and you dont mention that as a symptom.

What happens is that the extra air entering the engine makes the engine idle too fast. The computer responds by telling the idle air control valve to close, thereby causing idle speed to drop. When it does drop, the engine starts to idle roughly momentarily, because it is running too lean; to prevent a stall, the IAC is then commanded open again. This cycle results in a surging/ hunting condition because the computer cannot make teh engine idle smoothly at the speed that it is programmed to try to attain.

The leak can be external (vacuum hose, brake booster diaphragm, intake manifold gasket) or internal (wrong PCV valve with too large an orfice, throttle body deposits preventing the throttle plate from closing completely, EGR valve stuck partially open).

To find external leaks, a smoke generator works extremely well. Failing theat, you can mist a spray can of carb cleaner at sustpect areas, such as intake gaskets, looking for a change in idle speed. Try scrubbing the throttle body plate area with some carb cleaner and an old toothbrush to remove any deposits. Remove and check teh EGR valve for being open. Pinch off the power brake booster hose to see if teh idle smooths out.

I am sure you can find it, if you look diligently enough!

I hope this is helpful, if so, clicking on my accept button would be most appreciated... thanks!