When I went to the Armed Forces Day airshow at Andrews AFB back in the late '80s, they were using a C-5 as the "gate". You walked through it or under it to get to the tarmac and the rest of the static exhibits. I remember it being flanked by (I think) a KC-10 and a C-141, and it made them both look puny.
I've never seen one fly but friends who have seen them tell me they have a very distinct sound, like nothing else in the inventory.
}:-)4
Another distinctive sound is the KC-135 when it is using the water-injection system on takeoff -- they will shake the entire flight line area as they take off, plus leave a trail of black smoke down the runway that is amazing looking.