96 jimmy is making a bubbling sound in the front under the hood close to wall on the passenger side.

The heater core is at that location. When cooled down check the coolant level. If its filled you may have an air pocket in the heater lines that need to be bleeded. If so start the engine when cold and idle for 15 to 20 minutes. Then slightly loosen the top radiator hose clamp till you hear a hissing noise. If a single formed wired clamp is there replace it with a worm driven clamp. Now as I stated before loosen it gently & wait till the hissing quits and coolant appears only and tighten back down. The bubbling noise should quit. If not the heater core has a prblem such as being plugged up somewhat. If not attended to it will get worst and leak inside your vehicle

You have air in your cooling system, flush out your engine cooling system and refill. Replace the thermostat if not been done and the radiator cap. In the mean time check your coolant level and fill to proper level.

1996 GMC TRUCKS SSERIES JIMMY 4.3L 6-cyl Engine Code [W]
Cooling System, Initial Fill..........11.5 liters GM-Dex Cool 50/50 mix water/coolant.

Note: To much water in mix will freeze in winter and will boil. To much Coolant will gel when heated and restrict the flow of coolant throughout the engine.

Your coolant level is low, and you hear the coolant taking up the vacant areas....bubbling...due to low coolant.