Coolent Issues?

I have a 2000 Blazer with 36 on it and a week ago I started hearing what sounds like water boiling and poping coming from the pasanger side dash inside the car. It started at first to only happen for a few minutes in the
morning when first leaving, now its doing it more often now. It only makes this noise when you are pushing on the gas to speed up, and if in park and pushing the gas and reproduce the same thing. I did add a little coolent yesterday, but still hear it and I am not finding anything leaking from the heater core. Any thoughts or suggestions?

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