It’s always good to state which technologies you’re using, otherwise it’s guesswork for those who are trying to help you. Since you mentioned Unity now, I’m assuming you may be using Tiled2Unity or SuperTiled2Unity. In that case, the problem you’re having may be covered on the following page:
Extruding/padding your tiles is also mentioned there like @the-simian did in his talk, and if you want to use non-integer scaling factors and place your map at sub-pixel positions, this would be the way to avoid those lines.
However if you just do that the pixels will keep this slight “jittering”, so the preferred way to avoid it in my opinion is to use an integer zoom factor and round the map’s position such that it’s always aligned to physical pixels.