Ender 3 Pro - weird artefacts in printing

Got a weird issue - my printer is set up well and producing great prints as far as I can see, but I often get artefacts as you can see in the attached - somewhere on a print will be a vertical line of "stringing" which I don't believe is related to over extrusion.

I occasionally hear a click like the print head is clipping the previous layer when it travels to start a new layer, and I think this is what is creating this issue. The strings rub off easily enough but it's just strange.

Just wondering if anyone can point me in the right direction to understand this one, thanks.