The silicone fun factory vibes I own are all very, very quiet. So is the lelo liv. I think they're all effective but they're not going to be earth shatteringly powerful like a bullet or hitachi so... it depends upon what you consider effective.
I don't have any experience with quiet vibrators, but I do know that the reviews for all the vibrators have an indicator for the "Buzz" which means the level of noise. They also have "Vroom" to show how powerful the vibration is. I hope this helps!