Noise is typically from the gears in the reduction. The design and more important the gear precision is what makes a quiet gear train. Therefore a very high quality gear motor will be quiet.
Read the artifices here to learn more:
I have found high quality, quiet gear motors from surplus outlets for a fraction of their original price.
Problem with surplus is you never know what they will have in stock.
If you have the budget then buy a FAULHABER, else buy surplus looking for FAULHABER gear motors.
I have gotten a few for about $30 each (original price is over $100 each).
Next step for a quiet gear motor is how it is mounted and what it is mounted on. I have a number of Pololu gear motors. For the price these are nice and fairly quiet. I mounted one to a sheet of thin plywood and the noise was huge due to the plywood acoustically amplifying the noise. By changing to a 'floating' mount (rubber pad between the motor bracket and the plywood) the noise was greatly reduced.