presumably this is an i3 of some description ?

I have exactly the same issue. In my case it's because the extruder assembly on the x? (sideways) axis is not securely fixed to the guide rod bearings. So the whole extruder assembly actually rocks back and forth as it goes up and down.
This looks identical to the prints I'm currently getting.

Could you ave dislodged something while changing the extrduers ?