Quietness and effectiveness might be mutually exclusive. Any vibrator creates vibrations by oscillating a weight and most vibrators work by using an electric motor to spin an off-balance weight to create the oscillations. The slightest, I mean the very slightest play in the hardware mounting of the oscillating mass will create a rattling noise unless tolerances are very tight or dampeners are used, but... dampeners will also absorb the vibrations and reduce its effectiveness.
The manufacturing solutions would be...
1) Use no dampeners with sloppy tolerances = cheap/noisy product right out of the box.
2) Use no dampeners with high tolerances = more expensive but effective product, quiet at first but noise slowly grows as vibrations take their toll on manufacturing materials, e.g, plastics, etc.
3) Use dampeners with tight tolerances = even more expensive product, very quiet at first, but noise still slowly grows over time, in addition, product requires more power to overcome dampening effect to be effective.
4) Use inductive technologies instead of rotating motor to create oscillating mass = super expensive, no moving parts, quiet forever. Does this even exist?
Your best answer is to read the reviews and expect to pay more.