There are many o-rings between that cover and the pump housing. Over a dozen as I recall and any one of them can cause a leak. I would just buy a seal kit, take the pump apart, and reseal it. Deere uses two series of pumps. German design and USA design. You have the USA design. They came in several sizes and were originally expressed in cubic inches - not this cubic centimeter metric nonsense. You have a 2.4 cubic inch pump. Very common. The main shaft in it is supported by adjustable cup and cone bearings. So you take it off, check main shaft end-play before removing any bolts. If it is good - then no future bearing shimming needed. If too loose - you must tighten it up with shims.
The rest is pretty straight forward. All the USA pumps go together the same. The clacking you hear is likely your stroke-control-valve and seat. It is a normal wear item. Buy a seal kit, a new stroke-control-valve - and you will likely be fine.
I will note that Deere had many issues with the 8 piston nut-caps leaking on early pumps. They are all accessible on the outside of the pump. Deere's fix was to put a teflon sheaf over each o-ring before installing.
Deere German pumps do not have adjustable bearings and come in small sizes of .7 or 1.4 cubic inches (11.5 or 23 cc).
Deere USA pumps come in 2.4, 3, and 4 cubic inches (40, 50, and 66 cc)
Deere also made a four-piston USA pump that was just have of a bigger one.