In some cases electrical sourced vibrations can sound mechanical, I would try to eliminate a few things first.
Is this a DC motor and is it ±10vdc analogue drive?
If the above, I would take a close look at the tach if it has one, also it would help if you disconnected the analogue input and attached a ±9vdc battery box. It is simple to mock one up.
You can move the motor this way disconnected from the controller.
Does the motor feel rough when you turn it by hand?
If it turns out to be the bearing, they can be replaced relatively easily, remove all brushes and take the end plates off, careful when removing the armature as it will attach itself to the field magnets.
Some have said they have lost field magnetism when removing the armature, I have not yet experienced this, but if in doubt, slide a metal 'keeper' into the field as you remove the armature.
The commutator may need skimming and undercutting when you have it out, undercutting is using a special thin saw blade to clean the mica, or? from the comm. segments.
One can be fabricated from a ground down hacksaw blade.