I am not a self-proclaimed hydraulic expert like some...... But here is my stab at whats going on.
Is it a constant issue after the tractor has warmed all the way up or just when you just start working the loader when everything is not warm yet? Does it only do it at an idle or all threw the r.p.m range?
It sounds like cavitation to me. But either from cold, thicker oil not getting threw a filter/screens fast enough when cold, OR a filter/ suction screen that is in need of changing or cleaning not letting enough oil threw at all.
Again, I am not a self-proclaimed expert on hydraulic issues like some..... Its been my experience that when pumps go bad they either get so weak do to wear everything tied to it gets real slow....but then sometimes a pump will implode or sometimes pumps explode. Valves do funny things but your issues do not sound like a valve to me, but I am not there to listen or watch offending problem.
Again, I am not a self-proclaimed expert on hydraulic issues like some..... If it were my tractor, I would start by draining the oil, changing the filter or cleaning suction screen refill with some fresh oil and see what that would do. If that did not help, I would find out where and how to test pump pressure and go from there.