i think by know you would know if it was your headgasket you would have low water....burning oil....water in oil....or vice versa....lots of signs for it....i had the same water rushing sound in my 88 runner....when these motors start getting high miles and do blow gaskets....and the coolant rusts or whatever the case my be....small areas start to get blocked....and thats what your case is....you heater core is has some of the passages block (or restricted) which makes for high pressure on one side....and the water is backed up cause the heater core is restricted....flip your control the cold (blue area) see if the rushing sound goes away....if it does....there is your answer....

i actually picked up a 88 runner with the radiator and the heater core plugged....temp would be fine....but if i put any load on the motor (towin my trailer) it would over heat and the rush sound only went away when i turned the knob to cold.....replaced both radiator and heater core problem solved.....

hope this helps good luck....