slanw
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..
Firstly..
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..
In events:
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..