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

Have bleached the well and water system
Let's see if we can localize it.
Shut the water off at the cold water angle stop and unhook the supply. Now fill a glass of water from the stop. Does it smell?
If the answer's no then move on to the next step. If the answer's yes click on back and tell me about it.
Shut the water off to the house and pull everything from the cold water angle stop.m including the cold water supply, through the lavatory valve and soak them in a pan of bleach.
Good luck, Tom