[swedishandgerman]

Engine and in particular, gearbox mounts. The rubber goes brittle and out of shape with age resulting in the engine sitting too low and the gearbox on the wonk. That in itself isn't good, but allows the prop to be on the wonk too. When replacing - and this is a thing that often gets overlooked - you should measure things up to ensure everything is aligned

All of the above is after you've made sure the prop itself is sound