A few possibilities come to mind. It's possible that your dropouts aren't parallel, which is common on older bikes. That will bow the axle slightly when it's tightened effectively shortening it and tightening the bearings. Try re-adjusting, but leaving it a hair loose and see if it comes to a perfect adjustment when in the frame.

Pedaling can aggravate a bearing problem, especially on hills. The load from chain tension is greater than the load from axle weight, and might push a borderline situation over the edge.

In my experience ball bearings don't tend to make a creaking sound. Look at the hub carefully ans see if a seal or dust cover might be touching the axle. If you have a reason to open it, you'll see polished or scored areas on one side of a cone.

Lastly, your axle mounted RD wouldn't affect anything if, as cny-bikeman suggested, the locknuts on both sides are properly tightened.