We've noticed a rotten egg type smell coming from our bathroom. Upon further investigation it appears to be coming from the bathtub drain. Further, it only does this when the dishwasher is running and sometimes when the washing machine is running. You can also hear a noise (gurgling?) coming from the drain (maybe when the dishwasher/washing machine are draining?). We are on a septic system. The dishwasher and tub are on the same main floor and the washing machine is in the basement.
What could be causing this? It smells up the bathroom and works its way out into the other rooms.

Sounds like you have a partial blockage in your drain that's setting up a back pressure that forces a bubble of sewer gas back up the line. Have the drain lines checked and bcleared. Regards, Tom