Fluid gurgling noise near heater core on acceleration

2001 Chevy Blazer 4.3 151,000 miles

When I accelerate from stop, there is a gurgling water noise in the center of my dash. Once I get to higher RPM's it seems to go away or maybe the road noise masks it. I suspect it has something to do with my heater core, but there isn't an obvious leak anywhere. Any clue what this might be or what needs to be done? Thanks.