Moldy odor from lavatory overflow drain hole
I had the same problem. I thought it was the drain - there are stories all over the internet with people ready to rip out there bathrooms because the smell is so strong. In my case, you could smell in downstairs.
A plumber friend explained that if soap or a hair finds its way into the overflow pipe - mold can form (I have a 3-year old who splashes around in there each night). He said it was unlikely that it was the drain because that drain gets flushed 3 times daily with showers and bathes - so mold cannot form. The soap/hair gets in the overflow pipe can just sit there - and grow with the humidity of the bathroom.
I unscrewed the cap of the drain overflow - and low and behold - there was a thin coating of mold all over overflow PVC pipe.
I put bleach in the spray bottle and sprayed inside the pipe vigourously. I bought a LONG flexible pipe brush from Lowe's (or a plumbing supply store) and then scrubbed the pipe a good 6-8 inches down. Repeat 3 or 4 times. Clean the rubber gasket that compresses the pipe to the tub - and also the overflow cap. Make sure EVERYTHING is clean or the mold will come back faster.
In my case, this appears to be a 1x a year job - no idea why - it just is. Oh, and one more thing, make sure the screw that holds in the overflow cap is stainless steel. The contractor that installed the initial overflow cap didn't put a stainless one in - and the screw rusted to the overflow cap - causing major problems getting the cap off. This last time - it came out very easy.
