Toilets, Sinks, Showers, Tubs and Disposals - Smelly water from single faucet

In the bathroom, there are two identical commode sinks with individual Kohler faucets. My wife told me that the water from her cold water faucet "stinks" ... smells like "sewer gas."

I check things out and discovered:

The first half-glass of water from her cold water faucet does have a very distinct odor. I does smell like sewer gas or maybe rubber. After a glass of water is drawn, it clears right up. My cold water is no problem. None of the other water faucets in the house have odors ... only her's and only the first cup, or so.

Both faucets are relatively new, identical faucets. From what I can see, both have steel jacketed supply lines, however they connect under the base cabinet, so I can't see the actual connection.

I pulled the faucet strainer/aerator off, but it was relatively clean, with only a little grit. After replacing - same problem.

Any ideas?

A free guess? Any chance the odor might be coming from the drain -- when water is being drawn from the suspect faucet is any excess water spilling into the sink and drain? It would make sense if the order was coming from the drain system and not the water, especially when both faucets are fed from the same source... If it is the drain, try adding some peroxide and see if the odor goes away for a few days... Just a guess on my part, FWIW.

Only thing I can think of is a bad washer on the cold stem (or if it is single handle, a bad "O" ring in the cartridge). As the water sits in the supply tube, it is "contaminating" the water and then flushes out when you use that first 1/2 cup.

I am having the exact same problem. However, I recently replaced my faucets. The problem existed before with the old faucet as well. The smell is from the water, not the overflow. The smell only occurs for these two faucets, same bathroom, and only for the first 10 seconds or so of water.