Ringing. Ghosting. Wavy walls. They all are the same thing and they all come from vibrations in your printer. A loose belt on a printer can cause a significant degradation of your prints. When I see that I have wavy walls, I start by checking to make sure that my belts are tight and that my rollers function smoothly and that they are tight too. From there, I cycle the motors back and forth, one axis at a time. Make sure that the look smooth. The human eye is not great at approximating large distances, but fantastic at noticing small changes, such as a roller that has a slight bump in it at a certain point in the axis.
Once I am convinced that my printer is mechanically as good as I’m able to make it, I pull out my software tricks. I enable resonance tuning in Klipper and run the resonance tuning test part. I typically get different values for the x and y axis, as they tend to resonate at different frequencies. This does a pretty good job of removing any remaining vibrations out of the system.