https://www.consumerreports.org/cro/central-air-conditioning/buying-guide
http://georgebrazilhvac.com/blog/which-2017-air-conditioner-is-the-quietest
https://www.cooltoday.com/blog/how-to-know-which-central-air-conditioner-is-the-quietest
https://www.thisoldhouse.com/ideas/air-conditioners-really-are-getting-better
https://www.consumerreports.org/cro/central-air-conditioning/buying-guide
https://acousticalsolutions.com/reduce-unwanted-outdoor-noise-sound-blocking-fence/
https://www.bogleheads.org/forum/viewtopic.php?t=100286
Getting back to your OP, a compressor blanket is the best solution if the noise is coming from the compressor. The blanket goes around the compressor itself, not the entire unit, and it does not block air flow.
It also does not cause the compressor to run hotter, as the compressor is hermetically sealed and is cooled by the circulating refrigerant.
http://comfortgurus.blogspot.com/2010/0 ... nkets.html
FAQ:
http://www.brinmar.com/faqs_sound_blankets.html
Brinmar sound blanket
TRiCC utility cover provides significant noise reduction; proven in a field test by a LEED certified engineering firm: http://www.triccover.com/store/what-is- ... ity-cover/
This gives some useful information about sound wall construction:
http://www.fhwa.dot.gov/environment/noi ... epdown.cfm
There are pre-made materials for these types of applications, but I suspect the expense and appearance will not be acceptable to you or your neighbors.
http://www.constructionnoisebarriers.com/
https://www.angieslist.com/articles/tips-quiet-noisy-air-conditioner.htm
https://www.google.com/search?q=Compressor+insulation+and+mounts&oq=Compressor+insulation+and+mounts&aqs=chrome..69i57.798j0j7&sourceid=chrome&ie=UTF-8
https://www.houzz.com/ideabooks/29385453/list/5-ways-to-hide-that-big-air-conditioner-in-your-yard
http://www.quiet.org/documents/NotCool.pdf
http://www.acoustiblok.com/acoustiblok-residential/customer-project-testimonials/
http://brinmar.com/cad/sound_blankets.html
http://www.soundproofing.org/infopages/air_conditioner_soundproofing.htm
http://www.soundproofing.org/
https://www.acousticalsurfaces.com/blog/soundproofing/noisy-air-conditioner-2/
https://www.acousticalsurfaces.com/blog/soundproofing/how-to-quiet-noisy-ac-air-conditioning-compressor/
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https://quietfence.com/
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https://en.wikipedia.org/wiki/Sound_level_meter
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5102154/
http://www.daytonaudio.com/index.php/imm-6-idevice-calibrated-measurement-microphone.html
Compressor insulation and mounts