Smell from Shower Drain

I have an odd situation with the shower drain. It produces a sulfurous smell when you start running water down the drain. After about 30-60 seconds the smell is gone. It doesn't smell at all between showers.

It just seems backwards to me. I would understand if it smelled until you started running the water, but I can't come up with an explanation of why it would start smelling when you started running the water.

I've had some success with Clorox, but after a few days it starts again.

Any theories, ideas, suggestions?

If your water comes from a well, maybe some iron-related bacteria (IRB) from your well could be feasting on your cast-iron drain parts, if that is what you have. The sulfur odor is their harmless-though-obnoxious body odor.