After years of on and off squeaks I finally found what was making noise in the barn door area.

It was the support bracket of door lock actuator, that is riveted to the barn door. Rivet came slightly loose and braket sounded a noise occasionally.

The worse part in the story is that when I stopped to investigate, the sound vanished (since I opened and closed the door), but after driving a while the noise would return.

Replaced the rivets with stainless screws and locknut - no more rattle.