I would disconnect the remote hydraulic return line and route it to a clean bucket so you know that you have an unobstructed return flow. Operate some your troublesome remote functions and see if the squeal remains.
Air gets into your hydraulic system through leaks in your suction connections (suction line seals, and possibly pinhole leaks/cracks in the suction line). Leaking hydraulic filter seal is also possible. The root cause is probably due to a plugged pump suction screen in the reservoir. Do you see aerated hydraulic fluid?
The vent is necessary to expel air from the reservoir and is not part of the problem.