If its the dynamic seal inside the rack that's passing fluid then no, it wouldn't leak out of the rack. As Mr B says there isn't a lot of hydraulic pressure required to turn the wheels when they're off the ground so the leaky seal would still work ok in this situation. It's probably still leaking fluid past but, not as much so you can't hear it. When you load it up however, the seal has to be able to hold pressure to turn the wheels. If most of this pressure is leaking past it then it will make a louder noise and give you heavier steering.
Try the screwdriver stethoscope trick on the pump first and if that's ok then try the rack.
It could be one of the hoses starting to collapse internally but I wouldn't think it was likely.