You can always tell when a C-5 is approaching or flying by, even if it is above the clouds for fog. The high bypass fans on the engines make a whine unlike any other airplane.
When jumping from them, they descend and loose altitude as they cross the drop zone so that they don't stall. The airspeed limits on the T-10, MC and SF series of parachutes is at or below the stall speed of the big plane. When the last jumper is out, the pilot reves those big turbofans up ricky tick!