Hi Scotty. Thanks for the great videos.

2004 Ford Taurus Wagon, 3.0 Duratec with 222210 miles.   Previous scans showed lean conditions.  Ran fine in summer months, but last winter's symptoms returned and worsened with the recent cooler temperatures.  Rough idle, surging, missing on acceleration, poor gas mileage (although 27mpg on long interstate runs once I get to 65mph). Conditions persisted even after changing the MAF sensor.  So, suspecting a major air leak that the computer can't compensate for. 

I removed the plastic plenum/upper air intake manifold, mainly for access (changed heater core, plugs, wires, EGR valve, and EGR Pressure Feedback Sensor).   I reinstalled the plastic plenum and all the hoses and connectors.   Started the car and it immediately had a loud, erratic, but not steady knocking sound coming from the engine.  I'm 99% certain I didn't drop anything down into the intake manifold, although it sounds like it.  Could air leaks around the plenum cause knocking noises?  I have an 11 second video of what it sounds like if there's a way to upload it.  Would appreciate your expertise on my options.  Invest (gamble) on a new plenum?, or bite the bullet and pull the lower intake manifold, heads,...etc?   Thanks for any advice you can give!