Z Backlash could caused by these few problem:
1) Z axis wheels too loose or too tight. You need to disassemble the lead screw and moving the gantry up and down to check is the wheels are too tight (bumpy feel when you move the gantry). Try spin the wheels and see if they are too loose (spin freely).
2) Loose Spring couplet.
3) Slicing setting issue.
4) If you are sure it’s not slicing issue, it could be gcode corruption issue. sometime you may print the same part with same setting before, but it just don’t print nice. You can try slice again then print.
5) Make sure the height of both left and right side of X gantry is same when you measure from the leg.
There may be causing by some other issue as well, you’ll need to find the possibility and tackle it one by one. Maybe can search on the Facebook group or youtube, see how other user solving similar issue. Sometime it could be due t some issue that we don’t even thought off.
We once got a Ender 3 having similar issue, took us quite some time to find and tackle the possibly issue. End up we notice it could be due to the X motor plate not angled properly in 90 degree. So we try add a washer to one side of the brass nut to compensate the angle, printed a 1mm thicker Z motor mount to replace the original, it can be found in thingiverse.com.
Hope the information above can help.
Let share and learn together by letting us know how you solve the similar issue in the comments.