FDV needs overhauling at a guess!
The noise is pressure only just getting past a valve within the FDV, i suspect the hissing is from a seal in there some where?
Cavmad has this on one of his BX's and i've seen it many times before. I think it can be caused by low pressure, but everybody i know who has sat and changed the pump has found no difference, so i'd guess its an FDV overhaul!
But i know nothing about FDV problems 