To start of your gonna need a pattern and duplicate it.

Lower the opacity of one of the the patterns to about 50.

 

Then you're gonna make an new timeline

Frame rate 30 and 120 frames for about 4 seconds but you can play around with the amount of frames tho i recommend leaving the frame rate at 30.

Now you're gonna add the parameters that are these symbols.

There are 3 types of key frames for an endless and seamless loop you need the linear one .

 

Add one at the beginning and one at the end.

While having the last key frame selected go to the object tool and move the pattern in the way you want the pattern to go but lining it up with the duplicate pattern at the end

And now you got and endless looping pattern