If you really feel you need to burp the heater core then you might want to think about removing the expansion tank cap, setting the heater control on high, fully warming the car until the thermostat is open and hot coolant flows in the upper rad hose (don't worry, there should be no boil over even after 15-20 minutes of idling), then massaging the lower rad heater hose to force fluid and any trapped air through the coolant system and heater core and out the upper heater core nipple. I'll often massage the upper and lower rad hoses together in opposition to flush coolant back and forth through the block to move trapped air.

Maybe try doing this with the car parked or jackup up with the right side of the car high. The upper heater core nipple goes across the top of the heater core and connects to the upper right side as I recall, so any trapped air in the upper part of the core should now be in the upper heater hose and hopefully now be able to work its way back to the water pump on the return line behind the block.

I've not had such gurgling sounds in the heater core. Any chance your heater control valve is failing and somehow unusually restricting cooolant flow to make for an unusual sound? Also check that the small vacuum hose that connects to the heater control nipple isn't split or melted closer to the valve.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now