I'd like to know the inner workings of that actuator.

Given the lack of detailed information about it, I'm going to guess it's probably not using that good a quality linear slide which has some play in it rather than being preloaded, which is then being amplified by resonance probably triggered by some runout in the ball screw.
Your setup will be adding resonance due to it's centre of gravity away from the slide, so any play in the slide will be amplified.

What happens if you press down, and put a load on the slide as it moves?