Ringing with Ultimakers, what is the cause?

have been used quite a lot for the past three years

The problem, even if belts and such seem ok, might be that other parts start to wear out. The part that contributes te most to ringing is the motion system. Grooves in rails or worn out bearings are a classic way that introduces some freedom in the system that can result in rattle and ringing. A photo of the rails and bearings might help to identify wear. Remember that most bearings need some sort of lubricant to work properly and reduce the wear in them, which should be part of the maintenance cycle!

Another source for the errors could be found in the extrusion system. For example, there might be an increased play in the gearing as springs under constant pressure can lose some tension, resulting in too little pressure on the filament, which typically would show up as under extrusion if the pressure is lost completely. On the other side, the hobbled gear that pushes the filament might be worn, resulting in a lack of responsiveness of the filament movement, again resulting in under extrusion after some point. Once more, photos of this area could help to indicate wear and tear.

Another source might be the mounting of the printer - it has been shown beneficial to put printers on a slab of concrete that itself is dampened on a medium density foam, resulting in even dampening that is usually better than the rubber feet of many printers.