Hello Mehrdad,
Congratulations on your new RV-14A. Really beautiful!
When you power off the servos and run the trim motor, it is running full speed at a steady max voltage and there is no speed scheduling which affects the trim motor speed/voltage.
When you run the manual trim with the servos powered, speed scheduling is being applied, which is done with a pulse width modulated (PWM) signal.
Depending on the speed scheduling setup, the trim motor will run slower in some conditions, and this could make it sound different.
There is a limit to how much the servo can reduce the voltage to the trim motor and still have it move, so at some point it even pulses the motor to further reduce speed, so this is something else you might have heard.
If the pilot pushes the trim buttons when the autopilot is engaged (with auto-trim enabled), it assumes the pilot is trying to take over control of the trim motors, and disengages the autopilot.
Let us know if you have additional questions.
Thanks,
Steve