That's pretty unscientific. If I look online, I can find people that took measurements of the sound levels put out by DJI's products. Those report anywhere from 74-79dB (mavic spark, mavic air, and mavic pro) -
https://www.wetalkuav.com/ultimate-dji-sound-test/
For those of you that aren't familiar, here's a link showing some common comparisons (you can find other similar charts elsewhere):
Soft Whisper - 30 dB
Quiet residential area - 40 dB
Babbling brook - 40 dB
Light Traffic - 50 dB
Normal Conversation - 60 dB
Passenger Car - 70 dB
Vacuum cleaner - 73 dB
Alarm Clock - 73 dB
Coffee Grinder - 75 dB
Power Lawn Mower - 80 dB
Heavy Traffic - 85 dB
Noisy Restaurant - 85 dB
https://www.creativefieldrecording.com/2017/11/01/sound-effects-decibel-level-chart/
I live in a "quiet residential area", approximated above at 40dB
From the pictures in the article above it looks like the sound level was measured 2 feet away from the quadcopter. Let's use 74dB as the sound level of the quadcopter at that distance (the lowest measured value).
Sound level decreases 6dB for every doubling of the distance. That means the quadcopter would cross the JND threshold at 89ft away from me (possibly vertically).
If the routes are programmed such that the quadcopters fly at 100 ft altitude and then drop straight onto somebody's property I shouldn't ever hear it.
Note that the quadcopters used in this analysis are pretty small with essentially 0 payload capacity which wouldn't be much use for Amazon. Ok, so let's now take that loudest of the DJI products mentioned - still a small product. That's 79dB (5dB more than the previous analysis). It would now need to be flying 159 ft. away from me.
Let's assume I live in a typical residential neighborhood in NJ, CT, etc. and every house has .5 acres of land. That's a square, 147 ft on each side. That means that if I'm standing exactly in the center of my property I will hear the quadcopter delivering his package. If the quadcopters are still flying at 100ft, I would hear it overhead at 45dB (which may be enough to be annoying)
I'm assuming amazon's quadcopters will be larger still. And there will be one flying to and from each house a package is being delivered to (think of the shear number of these things).
It will likely be enough to be annoying.
As well they should
Data from Wikipedia on the health affects of noise on Humans: