Heater core?

When you operate the A/C does a puddle of water form under the truck? If not you could have a blocked drain and the water build up is causing the problem. The drain is not hard to clear but it can be difficult to reach. If the windows fog up and you smell a sweet oder and it smells like anti-freeze then it is likely a small leak in the core. They can be hard to find.

I am reading a note that says, "on "C" and "K" series, heater core may be accessible without removing heater case. Remove passenger's side door sill plate and kick panel. Pull passenger's side of carpet back enough to access bolt at rear of heater case cover plate. If all heater case cover plate screws are accessible".
If all heater case cover plate screws are accessible, begin procedure at step 3).
3. Remove seven screws attaching heater case cover plate. Remove screws and bracket attaching core to case. See Fig 1. Remove heater core. To install, reverse removal procedure.

What you are likely hearing is actually a common thing with an S Blazer. If you don't find any wet areas inside the vehicle or smell coolant it probably isn't a bad heater core. What you are hearing is air trapped inside the cooling system. The easy repair is called burping the system.
You can do this yourself easily. Park your vehicle so that the right front is as high as possible. (I use a convenient bank on the lawn!) Now with the engine cool open the radiator cap. Top the coolant up and start the engine. Watch the coolant level and it should drop a bit when the thermostat opens. Once that happens top it up again. Then put the cap on. Now top up the coolant reservoir as well. Drive around some and let the engine cool again. Repeat the burping and you should be all set.

Heater/Vent Module Replacement
Removal Procedure

Remove the instrument panel. Refer to Instrument Panel (I/P) Carrier Replacement in Instrument Panel, Gauges and Console.
Drain the engine coolant. Refer to Draining and Filling Cooling System in Engine Cooling.
On vehicles with automatic climate control only, cut (1) the outer layer of the A/C evaporator and blower module in order to access the service access cover. Refer to Blower Motor and Fan Replacement.
On vehicles with automatic climate control only, remove the service access cover mounting screws.
On vehicles with automatic climate control only, remove the service access cover.
On vehicles without automatic climate control, remove the blower motor resistor. Refer to Blower Motor Resistor Replacement.
Important: Mark and use the same mounting bolt (located behind the A/C evaporator and blower module) for the heater assembly during the installation process. Other mounting bolts are longer and will obstruct the operation of the temperature door.

Remove the heat/vent module mounting bolt through the blower motor resistor or service access cover opening.

On vehicles with A/C only, remove the right wheel house panel. Refer to Wheelhouse Panel Replacement in Body Front End.
On vehicles with A/C only, raise and support vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
On vehicles with A/C only, remove the front right tire. Refer to Tire and Wheel Removal and Installation in Tires and Wheels.
On the Utility only, remove the A/C evaporator and blower module heat shield screws.
On the Utility only, slide the A/C evaporator and blower module heat shield toward the front of the vehicle.
Remove the 2 lower heat/vent module mounting nuts.
On the Utility only, remove the A/C evaporator and blower module heat shield bracket.
Remove the bottom right heat/vent module mounting bolt from inside the vehicle.
Important: The heat/vent mounting stud has a flange on the inside and cannot be removed from under the hood. Stop turning the stud after the threads are off the heat/vent module.

On the Utility only, while an assistant inside the vehicle pulls the bottom right corner of the heat/vent module approximately 1 inch back, remove the 2 heat/vent module mounting studs.
On vehicles with A/C only, lower the vehicle.

On vehicles with a 2.2L engine only, remove the engine wiring harness bracket located at the rear of the intake manifold. Refer to Valve Rocker Arm Cover Replacement in Engine Mechanical - 2.2L.
Remove the heat/vent module mounting bolt located at the lower left side of the heater core to heater hose connection.
Remove the heat/vent module mounting nut located at the lower right side of the heater core to heater connection.
Important: The heat/vent mounting stud has a flange on the inside and cannot be removed from under the hood. Stop turning the stud after the threads are off the heat/vent module.

On the Utility only, loosen the heat/vent module mounting studs until the threads are free from the heat/vent module.
Disconnect all of the electrical connectors from the heat/vent module.
Disconnect all of the vacuum lines from the heater assembly.
Disconnect the heater hoses from the heater core.
Remove the heat/vent module from the vehicle.

To smell antifreeze inside the pax side the heater core is leaking-coolant goes thru it