Need help with a buzzing Glentek GM6000 (Fadal 6030)

This motor is on a Fadal 6030 (1998).
I just replaced the tachometer and the resolver (both new) on the X axis servo motor (Glentek GM6060-50 DC).
After reassembly, it seem to function fine, but when we mount it on the ball-screw and move it, periodically a buzzing noise comes from the motor.The noise only occurs when the motor is stopped. It is random and not related to any position or direction.

Everything seem to work fine, movement and positioning accuracy is OK. There are no errors in the system.
The ball screw moves freely and is able to be spun by finger tips.

When the motor is removed from the machine, we cannot recreate the noise by putting load/pressure on the motor by hand.
I have taken the motor apart multiple times trying to find what may be the cause, but everything seem to be in it proper place, move freely, bearings look and feel OK.

We have swapped it with the Z axis motor to check if there is a drive issue, but the noise stayed with the motor.

If anyone had a similar experience with a solution.... feedback would be much appreciated.