Sometimes the solenoid valves don't flip when they are supposed to (the changeover relay sometimes doesn't latch), which can lead to the valves locking closed in both directions and the pump straining against the pressure relief valve. Is that the rude noise you are describing?
The Smiths mini-pack is a very noisy, but highly reliable hydraulic pump. Try flipping the relay a few times to see if the problem clears.
Only suggesting it because mine did the same thing last week, but in my case being laid up for several years clearly didn't help!
Lauren