Neither of those videos are my MDX. My sound is much lighter and like yours, it actually sounds louder from underneath. It is refreshing to know that yours was just a valve adjustment. There is still hope for me! My independent Honda mechanic recognized the noise but said he wouldn't adjust the valves unless there were drivability issues. I will be doing the adjustment myself and hope to see a slight increase in fuel economy.
Please update if you notice any difference in your mileage.
'06 MDX Touring amazon/ebony