this very much sounds like a vics butterfly has come loose and is stuck in the intake runner (which was totally harmless in my case although i didnt think so at first when it happened LOL). this has happened to my engine few years ago. watch this video and compare for yourself and sorry about the color as i had the camera whitebalance set wrong.
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I think you face the fact that you will have to take the intake manifold out to remove the loose one and he other 3 so your engine is balanced. you will lose some bottom end torque biut the car is completely drivable. the work wasnt too difficult to do for me and a friend but it did take a whole day to do and do right.