I've seen it several times, and it has something to do with the Throttle position on the controller. It moves when I move the throttle lever on the joystick.

And I think it's this entry in the panel.cfg:

Window18=Throttle Synchronization

[Window18]
Background_color=0,0,0
size_mm=75, 200
position=5
visible=0
ident=12118
alpha_blend=0.60
;render_3d_window=1
;ignore_input=1
;locked=1
gauge00=2d!T01,                              0,   0,  75, 200