Hello everyone,
I am working on small electric motor design and I have some problems.
The main goals are:

• Noise level - as quit as possible
• Electric consumption - as less as possible
• High variance in operational temp -40...100
• Operational speed 4000-5000 RPM

I use 2 deep groove ball bearings with spring preload.
Also housing is not from Steel, so I use tolerance rings to compensate thermal expansion/contraction.
The TR is AN type (installed in the housing).
Housing rotates. Bearing are glued to the shaft.
Problems that I've seen:

• different noise level during operational RPM's. I tried to tight tolerances and I think it is not relevant at this stage.
  
  Even parts that have 2-4 microns tolerance still have different noise level.
• when I try to rotate motor using only my hand I feel some strange knocking sounds.
  There are more than 1 knocking per full rotation and it looks like it doesn't have constant location.
  My best guess, that TR deforms Outer ring of the bearing. Also, the knocking is much stronger under preload, than w/o it.

Questions to you guys:

• I am not sure if I need to do dynamic balancing of the motor. I don't have much experience in this field.
• What can produce knocking sound? Any thoughts about it?
• How can I calculate theoretical power consumption?
• What kind of books/courses/webinars can you advice the get deeper in electrical motor designing

Thank you in advance