Depends on the year. Possible problem with the SBC if it has it. Sorry I don't know the MY they were fitted, but hopefully someone will be along who does know.
You could also check the climate control system. 'Eggy' smell comes through the vents (especially when hot air is blowing). This is because the screenwash tank starts to release the smell (due to some chemical reaction??) and the smell gets into the cabin since the pollen filter is very close to the screenwasher tank. Very common problem in the W211 E-Class and its not a simple job to remove and clean the screenwash tank apparently.