In drone design, there's a trade-off between flying quietly and staying airborne for a long time. The Ion Tiger drone, which just completed a continuous 48 hour 1 minute flight, might be the breakthrough that changes this.
The reason for the trade-off comes down to fuel. Hydrocarbons (gasoline and the like) store an incredible amount of power in a small volume, which allows a drone like the MQ-9 Reaper to fly for 30 hours. Drone engines release this through combustion, a very noisy process. Quiet-running electric engines, on the other hand, are constrained by battery power, which typically limits flight time to only a few hours.