I hear it too,I think its the butteryfly valve in the exhaust,that gives the engine backpressure when its cold,helps it warm up quicker,kinda like an exhaust brake,the hissing is the air escaoing around it under high pressure,so Id say its normal until its fully warmed up anyway.
I have a 2000 also and it is partially because they did not put a catalytic converter on starting this year, the turbo pipe is a true 4 inches coming off the motor and it also is what John said about the butterfly keeping the correct amount of pressure not only when it is cold but also when warm to help monitor the temp of the turbo at the motor.
It is called an EBV Exhaust Backpressure Valve. There is a sensor on the motor that tells the PCM that it is cold. It closes the valve and raises the RPM to build heat in the motor by holding back the exhaust. For mor info goto http://www.ford-diesel.com and do a search on the EBV.
Yep, it is the EBV. I haven't quite figured mine out yet, as I thought it is only supposed to operate while the truck is warming up. Mine kicks in after it has been running hard sometimes. If you have problems with it, "ford-diesel.com" is the place to go. As there are many symptoms to the faulty operation of this thing. Good Luck