Probably the quietest vacuum I own is my Electrolux Ultra Silent (Actually a Eureka Product), a new machine, very powerful, only 8 amps, but the air movement is louder than the vacuum. My Miele is also very quiet. I would also agree with those who have posted above, the Original Electrolux G is very quiet, the Hospital G is even more quiet.
I also felt the Hoover Dial-A-Matics were quiet for an upright machine.