"You're probably smelling sewage gas escaping the drain's s-trap when the water runs. Keep in mind they put sulfur compounds in natural gas for safety, so it's chemically similar enough to sewage gas to smell the same. Sewage gas is flammable and toxic, so you might want to get that looked at. I had this same problem in an apartment I used to live in, which was a similar age. It turned out that the pipes were iron and had been filling the ventilation pipe with rust scale for years, which in turn prevented the pipes from venting properly. They ended up snaking the vent pipes, which fixed it to some degree, but the problem was still there till I moved. I'm not a plumber, but that was my best understanding of what the plumber told me."
https://boards.straightdope.com/sdmb/showthread.php?t=622918
