I've never smelt it but...
From what I've read it smells like, well, sulphur. Burning sulfur at that because that is precisely how you produce it. You burn a lump of sulphur, it combines with oxygen and next thing you know you are fumigating a huge wine cask. Seriously, they still use SO2 for that very reason. Have you ever wondered what "contains sulphites" means on a bottle of wine? Winemakers have used it since the dark ages, think 13th century, to curtail the growth of nasty bacteria or nasty by products in wine. The old fridge techs used to call it "volcano breath" since volcanos actually produce SO2 from the burning of sulphur, among many other things, in their normal activity. It just so happens to be an efficient refrigerant as well. Very efficient, part of the reason why the old fridges used to run only about 2-5 minutes at a time.
Wow, this got way off topic but I always love to talk about old fridges...
So, now I need a vintage GE cleaner to compliment my fridge! Just like I have a Westinghouse fridge and vacuum!