It is such a low note that it is more of a visceral feel than the noise like a jet or helicopter. I wonder what it sounds like when a "fleet" of these things are approaching a shore... I mean could it really be any sort of "surprise?"

We were about 2 miles south of the Point as this thing approached... and the rise of mist was visible for 5-8 miles out. I imagine Steve had no problem hearing or seeing the LCAC as it went into the bay.

On the other hand, the SEAL boats that drill from time to time are amazingly quiet.

Of course perhaps that deep sound is all part of the "shock and awe" that should be felt when an overwhelming force is about descend upon you. (thinking of the scene in "Apocalypse Now" with the helicopter calvary playing "Ride of the Valkyries..." )