Sewer-like smell

I received this question from a client of mine for whom I did a final inspection inspection about 10 months ago. Do you guys have any idea what the issue could be? (I waiting on him to tell me if it was the hot or cold water).
The house is plumbed with CPVC.

Trap could be drying out something logged in the drain prob. Hair remove trap clean and see if that helps . Tell them not to use sink for a day then remove and see if full of water

The gas has had time to separate from the water if not used daily. Usually after the dwelling is lived in for a while the odor will subside. Recommend water testing to determine if harmful or not.

From what i understand it’s when he turns the faucet on. It’s not at the drain. That’s why I’m stumped.

In my post I am referencing the potable water supply.

It could be an S trap, siphoning the water from the trap when inn use allowing the sewer gases in. Sounds like a plumber needs to be called regardless…

Does it light on fire?

It is a normal occurrence around here all the time.
Home that is vacant for a while , turn on the water and get the rotten egg smell. Run it for a while and it clears up.

See if this helps:

There are other possibilities…but, I’m thinking it’s from a magnesium heating rod in the hot water tank.

This only happens when the cold water is turned on and only in the 2nd floor lavatory. Very strange indeed. :-k

Was there a AAV under the counter top

No, house was brand new built to the 2009 IRC, plumbed as it should be.
My client said that the water coming out of the cold water made a lot of bubbles when it first starts running (which is when it smells)

Perhaps a foreign object was in the supply pipe during construction?
You should have caught that Will!