Hello! You may have very right. I am not totally sure about the N1 values I gave here, it was only estimate from me. Same bass sound does also appear when shutting down (moving fuel sw. from IDLE to OFF).
When going from app 70 %N1 to idle in air, it is nearly about the same bass noise, but not as load as in the start/stop situation. It is a mix off deep (but weak) bass and some vibrations.