There's probably a few ways to go about this, most likely some better ones from more experienced guys around here but this method works for me and doesn't require to many events..
Pick a Sprite on screen preferably on the layer you want to effect (just create a blank square and set invisble if you like) and add to it the "Sine Behavior".
In the Sine Properties set Movement to "value only"
Set everything to 0 cept' for Magnitude (100) and Period (say 2?)
* you can play around with Magnitude and period for desired result..
Everytick > System-Set layer Effects Parameter > The layer, The effect, Index (0), then > "Sprite.Sine.Value"
*replace 'sprite' .sine.value with your sprites name that you added sine to
* You'll probably need to do this for Both Horizontal and Vertical but they can both run from the same Sine.Value.
Hope that works okay for you..