Delta A319-114 (N316NB) on DL1391 (4/20/22)

Hello everyone! Today I caught a Delta A319-114 (N316NB) on DL1391 from Minneapolis, MN. The FOPP is amazing as always and this one sounds like a JetBlue A320 sound (normal sounding one). But there is an additional sound along with it. I wonder what it is and where it’s coming from. Even with the vortex generators, it still can be heard. This other sound only is found on the A320-211/212 and the A319-114 dating back from 1987 to 2003. So basically these are found on these aircraft with the CFM-56 5A series like the 5A1, 5A3, 5A4 and 5A5. Enjoy the video! 🙂