Strange behaviour x and y axis - corner of print Some background to the problem:
Several months ago I started getting poor print quality in one section of my prints (back left corner of bed) - I diagnosed that the plastic X axis carriage and the right Y axis carriage had distorted from heat over a period of 4 years running my Wanhao D4S, and the linear bearings no longer aligned properly.  I found a long gouge in the right Y axis rod and have replaced this.
I have since replaced all plastic carriages with aluminium to eliminate any potential distortion in the future.
Additionally, the 5 mm rods running in the X direction in both front and back of the printer also had slight bends in them, and I have replaced both these with 8 mm rods to enhance rigidity and prevent further bending problems.  With the bent rods there was a very noticeable vibration.
The issue I am now facing is the front right corner of test cubes I have been printing (see attached photo) are not sharp - they are okay at about the top and bottom quarter, but in the middle it is as if the X and Y axes are not travelling far enough to the front right corner, and the corner is cut off diagonally and tapers inwards slightly like an hourglass.  But as mentioned really only in this one corner.  The issue is most apparent where indicated by the arrows - although the others also exhibit to a small degree the same issue.

I have aligned all belts and pulleys, and made sure the belts are tight enough to &quot;twang&quot; but this does not seem to help.  All pulleys have also been re-tightened.
The printer also does seem to still have a vibration when moving along the Y axis, and is much louder than the X axis movement (which is barely noticeable).
When the belts are disconnected both X and Y axes move by hand smoothly.
Has anyone seen this issue before, or can offer a solution?  My next step was to check stepper driver current, although I don't believe this to be the problem as the issue is very consistent in its placement.
I think also overall, print quality in general is not as good as it once was before the initial troubles with the carriages started.
To help install the rear 8 mm rod, I unscrewed the two Z axis rods, but to the best of my knowledge they are in their original position.  They also move smoothly.
After one further check (stepper driver current/VREF), which I didn't think could be the problem due to not adjusting to cause the problem; it turns out this actually was the problem.
All stepper drivers have been reset back to correct VREF for their respective motors minus 10% as a safety margin and everything is printing well again.
I have no idea how VREF could have changed by itself, however thermal expansion on the boards due to constant use may be an explanation.
For anyone else experiencing similar issues and who have discounted the stepper drivers as the problem, I suggest resetting VREF to see if that helps.