that sound is oil passing via the relief valve its normal.

if it happens continuously that is cavitation which can be bad. air in the system will also cuase it repeated fully stroke the rams in either direction for a few seconds each way to bleed the system keep the oil topped off. if after you bleed it the noise reduces then entrapped air was the cause. also why it important to use a hydraulics oil not engine oil in the systems as they have the additives to help with dissipating the air that get entrained in the oil while the system is working.

vane pumps are known to be noisy so are gear pumps the 2 most common types. and cat used vickers vane pumps on alot of applications. sperry-vickers became part of eaton hydraulics.