Black edges on overlapped raster images in QGIS

I'm beginner with QGIS and I have task currently to make some GIS map/data.

I downloaded 3 satellite images from Landsat by USGS (multispectral, georeferenced) and I did overlapping of images using merge tool. I checked with x - no data value and load into canvas when finished. When I used merging tool I've got situation as you can see by the picture.

http://ift.tt/1Gn5K9t

Images has been overlapped perfectly but with strange black lines on images edges (where overlapping was made). Does anybody can help me regarding this issue? Have in mind that i'm a beginner :)