Heater Core Overheating???

[wldcat10]

I think my Heater core is over heating (not to sure)? Once in a while, I will drive to the store or to my parents house couple miles from my home in my '70 lwb and a bubbling sound will come from behind the dash on the passenger side. It doesn't happen all the time though, just sometimes. I'm thinking it's the heater core. What do I need to do/fix/replace in order for it to stop making that bubbling sound? Any help will be appreciated, thanks!