Jurgen,  One source of drive noise which is sometimes overlooked is
a lumpy or stiff V-belt.

I learned this while adapting a jack-shaft to an industrial sewing
machine (to slow the stitch rate down to something I could manage).
I had purchased a cheap belt at the local auto parts store for the
jack-shaft reducer.  During operation, this belt made a lot of noise,
and caused the sewing head to vibrate.

I discovered that the belt varied in thickness across the V section
in different parts of the belt.  This resulted in the belt tension
changing with position of the belt as it moved over the pulleys.
I purchased a high-quality belt through an industrial sewing machine
shop, and the noise and vibration disappeared.

In general, I think noise reduction requires close observation of the
character of the noise, followed by simple experiments to confirm what
might be happening.  Here are some examples of simple observations and
experiments:

 - Listen critically to determine the periodicity of the noise.
Relate the period to the drive components.

 - Loosen and tighten the belt tension to see if it changes the noise.

 - Lightly hold a screwdriver shaft against a housing to hear it
buzz with vibration and to determine the axis of vibration.

 - Reduce the motor drive voltage (with a Variac or a transformer)
to see if that reduces 120 Hz hum.

 - Loosen and tighten the motor mounts to see if sound is being
conducted from the motor frame to the instrument.

 - Change the loading of a shaft axially or radially by pressing on the
shaft with a lubricated piece of wood, to see if bearings are noisy.

 - Use a mechanic's stethoscope to listen for noisy bearings.

John Rhodes
Vancouver, WA