Well, I'm not even sure it's a pressure problem. I was just in the kitchen getting another cup of coffee, the heat was on, the sink was gurgling, the heat cycled off, and the gurgling stopped. Now the outlet in the ceiling in the kitchen/dining area does allow some air to blow directly into the sink. I just cannot imagine it being enough volume of air, or pressure, to cause that gurgling. And I also cannot understand why this just started happening earlier this month. However, it's only happening when the heat is on; not the cooling (volume of air being moved feels about the same). We had a complete new HVAC system installed in May of this year, so other than very brief testing, the heat wasn't used until this month.
But anyway, I'm just happy to know I don't really have a serious problem.