might be cracked head or blown head gasket...
Can you locate the source of the hissing?
Is the vehicle otherwise overheating?
Did you have any overheating episodes right before this started?
Any evidence of antifreeze leaking on the ground?
How's your radiator cap?
Have you pulled your spark plugs to see if there's any indication of coolant or contaminant on them?
Does the engine ever run rough after it's been parked for awhile, then smooth out after a few seconds?
My '94 was loosing coolant through the recovery tank every 60-70 miles. It would hiss and gurgle when I parked it. Turns out it was one warped head, and one cracked head, the end result of a seized thermostat and severe overheating (wife kept driving). I was seeing NO visible loss out the tail pipe, and NO visible coolant in the oil. I took mine to the dealer and had them do a pressure leakdown test on the cooling system, and had them test for CO in the coolant. The second test (for CO) told the tale in my case, and now I'm in the process of putting the motor back together.
One wise old (experienced) machine shop guy told me that the 4.0L is prone to cracks above the combustion chamber. This has been reported by others on this forum. Search on "overheat" or "cracked head" or "coolant loss"... (You'll find that you're not alone.)
You might drain and remove the radiator yourself before taking it to the shop. Not too difficult - took me a couple of hours. Then look in through the hose necks and see what kind of shape the cooling tubes are in. With 105K miles, mine were almost new looking, so I ruled that out of the list of potential problems.
Let us know what you find, and I hope it isn't bad news...