Hey fellas, I get these peeving little seams in the middle everytime I use subsurf after mirroring a mesh. Three days I’ve tried to fix this on my own but now it is time to admit defeat
My pivot point is on the selection, on the lower image, I tried switching it but it didn’t seem to have any notable effect
With subsurf
Without subsurf
This makes my Blender sad 
Looks to me like you have faces on the “inside” of your model running down where the seam it. In the mirror modifier panel there are icons for disabling the modifier in edit mode, disable it and go in and delete the faces on the inside of the mesh.
Randy
I think you have internal faces, delete them and the problem will go away.
Thank you, it fixed it =)
Now I understand why people cut the mesh in half before ‘mirror-modeling’