If you grind oily dark roasts, the oils stick around in your grinder and go rancid. That's what you're probably smelling. You just need to clean your grinder so

If you grind oily dark roasts, the oils stick around in your grinder and go rancid. That's what you're probably smelling. You just need to clean your grinder some what regularly. This isn't a problem for medium-lighter roasts and water added with RDT doesn't really have any effect in practice.