We have a rotten egg odor coming from the sinks in master bath when cold water is turned on. It goes away after the water runs a few minutes, but is there each time the cold water is turned on. Not from the hot and have a tankless water heater. Also have a water filtration system and the problem is still there. Have bleached the well and water system. Have had smoke test and a peppermint test done. Two plumbers stumped - Please help