Hey @Xokks!
Have you tried lifting them up a bit and then pressing “End” to line them up flush with the ground?
Other than that, check your static mesh’s collision and make sure it’s also flat on the bottom.
It’s not that you’re turning on collision- it’s that you’re turning on simulate physics and there is a tiny bit of overlap happening somewhere between the ground and the base of your model. 
Hope that helps!