Hi AnthonyGA,
If you have a look at :
Arrangement for minimizing buzz saw noise in bladed rotors - Patent 4732532 it mentions several of the reasons for the buzz saw noise.
"This noise arises from the production of unequal shockwaves during the rotation at sonic or supersonic velocities of a bladed rotor, the dimensional characteristics of whose blades are subject to manufacturing tolerances."
The L1011 air conditioning was the noisiest I've come across. I bet most of the cabin noise came from there - it was just as loud with idle thrust.
(Thanks for the warning about 411A - I meant to explain that the engine noise on the L1011 is so quiet, that the only perceptible noise in the cabin was from the conditioning - louder in descent with idle thrust when the HP air was used as the bleed source.)