Make sure that everything is mounted tightly and that the panels
the pump and actuator are mounted on are sturdy. Its possible that one of the panels
could be "oil canning" if it only happens when it is working hard.
I would start by removing all the pump mounting bolts and checking that the holes haven't worn. You might think about mounting the pump on a panel and then shock mounting the panel to the original mounting point.
Its a possibility that the hoses are moving and causing the noise
. Hard to say because I haven't heard what it sounds like.
Check that there is no movement in the quadrant on the rudder
The actuator needs a very strong mounting shelf/bulkhead. Any slack in the mounting holes/bolts may cause noise when the working gets hard.
Trying to absorb or block the noise may work, but eliminating it will give you the best results. You may have to resort to both methods.