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The Mavic Pro has bigger propellers to produce more lift because it is heavier meaning it needs to push more air to keep it up meaning it is indeed louder!
To read more about how the 2 drones compare you checkout my detailed comparison right here.
The Mavic Pro is heavier, and therefore needs to generate more lift, resulting in bigger props and more power needed for the motors. Compared to the Spark, the Mavic Pro is louder, but DJI has released a Mavic Pro Platinum, which is considerably more quiet than the Mavic Pro standard.