'96 Sunfire 2.2L Pings and has rotten egg smell exhaust

The head was replace in its entireity 9k miles ago. I have always performed excellent routine maintenance. I do have a K&N air filter and a flowmaster baseline performance exhaust.

The car will ping under moderate engine load regardless of external conditions (rain, heat, etc.) When the engine is cold. Under either heavy or light load the pinging does not occur. I have always pumped 89 octane (R+M)/2 method gasoline into it. The exhaust always smells like rotten eggs (faintly albeit). The pinging recently developed. Anyone got a suggestion for where I should poke around?