When you operate the spools, are they doing anything? for example ,moving a ram. If you are just moving the levers without working anything I would suspect that the noise you hear is simply back pressure from the relief valve rattling back down the pipes. Which is nothing to worry about.
I am a great believer in the saying" If it aint broke, dont fix it", so if it all works and you have good seals around the Big filter case so as not to let in air, as Tom suggested, then leave alone.
Dave