You can look at the various DIY threads in this forum section, where people use DD hubmotors in the frame, and then if you drive it with a sinewave controller, it should be as silent as is possible, depending on the drivetrain you use to pass the power to the wheel. A belt will be quieter than a chain, but lossier and not generally shiftable for different gears.
Some of them are indexed here:
https://endless-sphere.com/forums/viewtopic.php?f=6&t=38553