"A visual inspection of the trim panel showed that a spacer acted like a bridge, allowing vibration transmission from the door module to the trim panel. In this case, structural vibrations were transmitted from the door model to the trim panel and were further transformed into acoustic energy that radiated out as sound. "
Design tools quiet automotive noise: Part 2 - an electric motor application | Automotive DesignLine
PC, this link from the original article is fascinating, and indicates that if you stop the transfer of vibration from the metal panels to the interior trim, you can really quiet sound quite a bit. This is very different from just plastering every interior surface with heavy mats like Dynamat or Second Skin.
"The results show that there was very little sound coming from the motor directly. The sound was emitted from the door module. In other words, the motor was the source of vibration energy, but the source of sound was the structural vibrations of a door module. Therefore, any attempt to modify the design of the motor will be a waste of time and effort. The focus should be on isolation of the vibration transmission from the motor to the door model.
The results demonstrate that the [electric window] motor is the source of energy in the system, but not a source of noise. Most of the noise is produced by structural vibrations of the door module. The motor then makes a door module vibrate, which acts like a speaker."
Pretty interesting and practical stuff.