Yeah, sounds like a tiny hint of H2S (hydrogen sulphide). I grew up with water like that. It's annoying but basically harmless at those concentrations. (If you can't smell it, you're in danger. Crazy stuff.)

From wikipedia FWIW: "Hydrogen sulfide can be present naturally in well water. In such cases, ozone is often used for its removal. An alternative method uses a filter with manganese dioxide. Both methods oxidize sulfides to less toxic sulfates."

I seem to recall that filling a jug/canteen about 80% with water, closing and shaking vigorously, and then leaving the container open to the air for a while would tone down the sulphur smell somewhat.

Funny thing is, we got used to it. When I moved to a city, the smell of chlorine in the water would make me gag.