Block Combinations Blocks that shake (like a car motor)

I post it as a quizz, as I don't know if a such behavior has already been quote with these elements.
My son showed me one of his unachieved level and pointed out a strange behavior somewhere. So I have put it on a box to show you and ask: do you know what lead the motor to shake? OK... There's a slider... But what else?
Here it is:

 

Oh sorry. I should have specified that the two opposing motors are much more about cosmetic, such as the "light" and other stuff. Did you try it ?

 

For no confusion with non important cosmetic stuff :

 

I admit that I don't have a list of all possibilities to achieve a such "shaking" behavior.
@richardfu_

Spoiler

Good point with the eye. But it can be achieved with something else than the eye which best suits a motor fonctionning What surprised me is if I put an eye on a slider with rail it shakes in some ways. With more slider+eye that shakes much more even if there is distance between them

I agree about the noise. That's why I've found appropriated to mimic a car motor However I don't see any practical benefit in using it in a level...

 

Well done friends! That was not so hard
Just tiny funny moments I hope