Here's the thing. Amd doesn't really have a "north bridge" so to speak. Most people around here are aware of "two." The Cpu-nb, which is the Integrated Memory Controller (IMC). It resides on the cpu die itself and is cooled just like the cpu. There is no temperature reading for this since the cpu is the same temp.
With that said, the "nb" you speak of if the Amd chipset. I don't believe it has a temperature measure, but I'm not the man to ask about this. The issue you've heard is simply a contact issue between the chipset heat sink and the chipset core itself. You can fix this by pulling the heatsink, cleaning the residue off and reapplying a thermal interface material (TIM).