Odor under the sink

We recently bought a new home (June). Starting in September we began smelling a strong odor in the Cabinet under the sink in the downstairs bathroom. The odor is similar to Mold.

We have nevered stored anything but dry linens in this cabinet.

Our builder has done the folllowing to address this problem - without successful results:

1). Changed the vent cap under the sink
2). Changed the trap under the sink.
3). brought in someone with a device to measure Moisture in walls and floors. None was detected.

Does anyone have any idea what could be causing this odor ??

Thank you for your help.

[Grumpy]

Moisture could have gotten trapped in the walls during the build out. I'd tear out the wall floor cabinet anything...