at around 2000 - 3000 feet is when you're extending the slats and thats when the engine fan blades accelerate about 7 percent to approach idle.
the engine could also be revving up to maintain slats speed usually close to 180 knots. i avoid this by selecting a lower speed. i wish i could be my own passenger some day and feel the difference.
the sound others are describing when the slats extend is the slat hydraulic servo which you can hear from inside, but impossible from the ground