[mylife]

While living in China, Dunlop ate rabbit heads, pig brains, scorpion and preserved duck eggs - known as 1,000-year- old eggs - whose oozy black yolks and 'noxious aroma' caused her flesh to crawl, made her feel sick and left a toxic black slime on her chopsticks.

[Zhang Fei]

I am gonna guess that the noxious aroma comes from the lye used to prepare the eggs. (The traditional recipe included wood ash - again the traditional way of making caustic soda):

Even though the traditional method is still widely practiced, modern understanding of the chemistry behind the formation of century eggs has led to many simplifications in the recipe. For instance soaking the eggs in a brine of salt and lye (Sodium hydroxide or Sodium carbonate) for 10 days followed by several weeks of aging while wrapped in plastic is said to achieve the same effect as the traditional method. This is true to the extent that egg curing in both new and traditional methods is accomplished by introducing alkali hydroxide ions and sodium into the egg.