I hove 1998 jimmy heat works good ,when i change

The switch that controls the defrost, heater and vent settings have gone bad and needs to be replaced. Auto Zone should have that part or you can do the Salvage Yard for around $10.00. The switch that controls the setting has a diaphragm and when it fails, the vacuum from the air lines starts to make the sound you hear and there is not enough pressure to engage vent doors. It take about 20 min to replace from start to finish and your only going to remove two screws from the dash over the instrument panel, the trim panel around the stereo and climate control just clips in place. You don't have to remove the trim panel, it's to much of a pain. the climate control is held on by 4 bolts and the vent setting is held on by one bolt. Once you have the climate control panel unbolted from the dash, disconnect the vent control vacuum lines and then replace the vent control switch. Once replaced, put everything back together in reverse the way you took it apart. Good luck and check that the new vent control works before you put everything back together, just be take caution when you have the engine running to check.