Approaching this kind of problem is a process of elimination.

Initially Flash sync seems most likely but the effect would be across a landscape frame. It also happens when the flash is not in use. So whatever it is happens in-camera and is not mechanical.

Any kind of lens issue would neither be as straight as those displayed nor as consistent.

After that it's something electronic... Anything in the digital domain would yield a binary failure (any effect would have a sharp edge.)

That leaves the only optical analogue component which is either the sensor itself or that the a/d converter is at fault.

You could do further testing, try looking at 'Live View' for example and see if the problem exists there but realistically whatever it is beyond what you know currently is not going to be fixable without the camera taking a visit to the service centre.

By 'just expired' if we're talking a few weeks then in many territories the manufacturer will honour the warranty as a goodwill gesture. Give it a try.