Sorry I didn't give a lot of detail. I have a Whirlpool WHES33 LE33 softener. I have historically used Iron Out (now SIO) every 3 to 4 months as directed (1/2 cup dissolved in a gallon of water, dumped into the brine well before regeneration). I have never had any issues other than maybe a slight iron out odor for a day or so after regeneration. The last time the softener regenerated, maybe a couple weeks ago, I added the SIO prior to, and shortly after regeneration I noticed this strong odor and taste. It doesn't really smell like SIO, it's hard to explain what it smells like, other than it's really nasty.
As others have mentioned, it only occurs in the cold water, not in the hot water ( I have an on demand water heater, no tank) and the smell in the cold water dissipates quickly if left to sit in a glass, which seems to indicate it is a gas.
I've probably regenerated the softener 5 or 6 times since this started, with no apparent drop off in the odor. When the softener is regenerating, the odor is not present. My untreated water has an earthy, metallic smell, but nothing like what I am experiencing now with the softened water. The softener does still appear to be functioning normally otherwise.
Thanks.