What you're describing really sounds like a vacuum leak. Those are typically louder at certain throttle positions, wherever the engine is drawing the highest vacuum. The fact that you're hearing it more with a less restrictive air filter, and you're hearing it so much at the fender intake, indicates that it's sucking air somewhere within the intake tract, anywhere past the air filter box. Could be a bad gasket between assembled intake parts or poor connections on the boot from the filter box to the throttle body. To eliminate external sources (as opposed to within the intake tract) you can lightly spray carb cleaner on every vacuum hose connection in the engine bay, while the throttle is being held in a position where it makes the noise. If you have a leak anywhere there, it will momentarily go away with the carb cleaner. The you can do the same at all locations where the intake system parts bolt together, like at the throttle body, etc. If it's a bad seal or gasket you'll get the same result.