What is propeller / engine sync and how does it work?
I don't know how it is achieved, but here is the reason:

If you have two sources of the same noise, with nearly the same frequency, the sum of both noises will be a noise of slowly increasing and decreasing volume. It's called beat and can become very annoying.

The math says

$$\sin(2\pi f_1t)+\sin(2\pi f_2t)=2\cdot\sin\left(2\pi \frac{f_1+f_2}{2}t\right)\cdot\cos\left(2\pi \frac{f_1-f_2}{2}t\right)$$

To demonstrate this, there you see two sinus-tones of almost same frequency, and what happens if you mix them:



The blue curve is of frequency $(f_1-f_2)/2$.

As example, one machine running at 3000rpm and one at 3030rpm results in a noise which increases for one seconds before it decreases within one second again.

As said, I don't know how it is done, but synchronization must be done very precisely to avoid this beat. 

Edit:

Here is what happens if the two noises do not have the same volume. One of the curves has three times the amplitude of the other. The envelope is not a pure sin function, but the blue function fits it quite well.