Hmm I think the Electrolux Model G is tops for quiet. Also some Panasonic uprights are fairly quiet. I think it was Panasonic that even had a " Library quiet" series. Although I doubt they could have actually been used in a library full of people. LOL
The DC01 is probably one of Dysons quietest cleaners however its not a candidate for the question asked.
As for quiet are we talking most quiet on full power or can we include quiet on lowest power?
If I need to vac really late at night I tend to go for the Miele S5210 on its lowest 300w power which is definitely the most quiet cleaner I own and at 300w its just enough for a quick tidy up and surface litter removal.
Full power wise the Vactric Silent Q is a contender however thats only 300w itself LOL. However Id hardly call it a performer.
An Electrolux Loaf cleaner like the 330 or the 500 are good performers and quiet as is the upright Moulinex Major but they are slightly noisier than the Silent Q.
Uprights: Sharp library quiet, Windsor Sensor (Sebo G) Sebo Electronic 370, Windsor versamatic, Lindhaus Healthcare pro i'm not sure which is the quietest of them.
Canister: The lux G takes it it's even quieter than my Miele Silver moon
Central: Drainvac special edition, It's noise level is comparable to my Sebos
My Electrolux UltraSilencer Green canister is extremely quiet on the lowest setting. You can talk on the telephone while using it! The sound of the air rushing through the carpet nozzle is louder than the motor itself. It does not have a power nozzle though. Another good question would be "which is the quietest power nozzle for canisters?"
Also, throughout the years, Japanese made Sanyo canisters and Transformax Stick Vacs are some of the quietest vacs I have ever used! Sanyo makes the Hoover Duros, so I wonder if that is one of the quieter contemporary power nozzle canisters.
I would say the XXX Lux or the Lux Intelligernce/Guardian Platinum followed by Sebo/Windsor and Miele Capricorn/Earth. Lindhaus falls in the list somewhere too.
the sanyo dirt hunter I had one for years and nobody could tell I would be vaccuming at 4 am and it was so quite that the hoover empower hush mode wasn't worth putting on