No,you should not hear hissing from your gas cylinder. There may be a slight noise if the cylinder is in use and your dispense equipment is being used. This is just the sound of the valve allowing gas to replenish the kegs that are in use. If the gas bottle is full and not connected and the top valve on the cylinder is properly closed and hissing can be heard then probably the cylinder is faulty. Put it to one side in a well ventilated area and contact your supplier for a replacement. If the cylinder is connected to your gas system and hissing can be heard then quite possibly the washer that fits on the high pressure hose gas connection or the main reducing valve if it is the type that fits directly on to the gas cylinder may be damaged or missing. Check this and if you have been instructed on how to fit or replace these washers replace them. If the hissing continues ventilate the area possibly leaving the cellar door open then turn the gas off and ring technical services.