Not an Bus pilot but this question comes up regularly, wondered myself at one point having flown in an A320 a few times. Actually your guess is on the right track, not fuel though. Most likely it was the PTU, Power Transfer Unit, in operation. It transfers hydraulic power between hydraulic systems. If one system is lower than the other it automatically kicks in. If it was a hacking, sawing sound that's what it was. Routine stuff.