CR10S Pro V2 Z axis bulge

[squaregoldfish]

Every print I make has a small bulge at the same Z position (see photo). The rest of the print has no issues. I assume there's something not quite right with the Z axis stepping at that position, but I'm new to 3D printers so I don't know what I should be looking for. Can someone give me some suggestions please?

Thanks,
Steve.