Mr. Lappos:
Telling a student who is simply trying reconcile your explanation with the flight school textbook answer that he is "spouting gobbledegook" is condescending and unconstructive.
Every student reserves the right to question and not accept answers on blind faith, even those coming from household names with monolithic credentials.
You made two points:
1. ETL transition vibrations are due to BVI. Thank you for setting us straight, unfortunate that the textbook is so misleading.
2. There is no blade slap because the velocities aren't high enough for transsonic shock formation. While true, this did not answer the question, there are still several other mechanisms present to generate blade slap at subsonic velocities. Mr. Crab helped shed some light on this by mentioning the angle dependency, thank you Mr. Crab.
Anyway, my name-calling was equally unconstructive, and I retract it.
-1B