Look for DC motors designed for 0 cogging, fat wide diameter bodies, balanced, and run the at low RPM. You may add a heavy flywheel to further lower noise.
Your drive power supply should be linear if you are worried about high frequency noise coming from a PWM drive.
For low rpm and low strength, you may also look at 3 phase or multi-phase brush-less motors fed with low power fundamental square wave or sine wave. Something similar to what you would find in old 3.5inch floppy drive motors. Slow regulated speed and silent, well the better ones were silent...