Musty Odor From Sink Drain

Hello all,

We are in a six month old rancher with a walk-out basement. Recently, the sink drain in the main level 3/4 guest bath has been giving off a musty odor when we run water. It is definitely coming up from the drain, and it is not the water itself.

The sink is used several times a day for hand washing only. The shower is used three or four times a week. All the plumbing under the sink appears perfectly normal. The drains work fine. There are four other sinks in the house, also all on the main level. None of them are giving off the musty smell.

There are two floor drains in the basement. The closest by far receives the condensate from the AC, so I know its trap is full. The other one is at the other end of the house. It is probably dry, but no odors come up from it. We are on a septic system. The water is municipal The house was plumbed by one of the best known companies in the area.

Thoughts?

Clean the sink drain and the overflow on the smelly sink.

Someone is probably brushing their hair over the sink and there is a stanky hairball under the pop up stopper.

Do you know how to remove the pop up stopper?