Cockeyed,
I can not look at the parts breakdown on the AGCO sight for reason so I am uncertain as to style of pump on this model. Is it a radial piston or axial piston?
Axial piston: the pistons run parallel to the drive shaft Usually what breaks on these is the slippers. When these break they usually destroy a wear plate on the swash plate and send metal filings into your bearings etc. Causes can be from excess speed, sudden pressure changes, case pressure higher than inlet pressure, preload spring(s) broken or fatigued from old age, heat etc...
Radial piston: I am not as familiar with but would suspect over speed, or over pressure could break them. Depending on what loads them against the cam this could also have broken causing it to knock.
good luck and keep everything very clean during reassembly.
Roy