It’s the takeoff. Below is a great example. Just like the takeoffs I had in my AA 757′s to/from JFK on my latest trip.
It’s that whiiiirrrlllllbzzzzZZZZZZZZZZZZiiing sound. It’s just like candy. And the harmonious ZZZZZbuuuurrrrinnnnng power reduction. Love it. Give me more power!!! Gives me goosebumps every time. Turn up the volume to an embarrasing level — this one is great.