the hissing is the exhaust back preasure valve. I disconected mine. its a 2 wire plugin found on the under side of the turbo compresor housing.
the missing might posbily be a bad injector or a bad conection on the under valve cover wiring harness going to the injectors. might have an injector buzz test done.
the dealership should be able to recalibrate your speedometer. if not you will have to get some thing like tru speed or speed wizard to compensate for it. any off road shop should be able to hook you up with one of those but id check the dealer first.
to powerstroke.org