My car makes a high pitch "hiss whine" near the waterpump as well, only when hot. Is yours a Delco pump? My dad's 1992 Sable used to make a noise like that from a bleed valve in the EGR system (lots of Taurus/Sables do it), I am unsure of the cause of the hiss noise on the 1998 Mystique. If the knock is slight it may be OK, my sister's 1995 Mystique does that from somewhere on the pass side of the v6, not the main bearings.
