Shower - drain smell and stain
I'm having a problem that I don't understand. I'm in a small apartment building (4 units) and recently my bathtub drain has started smelling bad, sewer gas I assume. It seems to happen after I run water in the sink briefly (not so much if I let it run for a minute or more) but it also happens at random times because I go in the bathroom and it sometimes just smells. I do occasionally get a blocked drain (I have long hair) but it's flowing fine now and draino always seems to fix it.
There are two other weird things happening. One is that the toilet and kitchen sink drain both randomly bubble, and sometimes that smells but not usually. It never overflows, just bubbles loudly. The other thing is that I keep getting brown stains on the tub from the overflow (I think that's what it's called, the thing that has the lever for the tub drain), like some fine brown mist is being forced out of it. The stains rinse away when I shower but they are resistant to being wiped with a cloth, they need a lot of water to go away. They keep appearing more frequently, maybe once every other day, as does the smell - every day.
Any ideas as to what is causing the smell and stains? This is actually and old house that was made into apartments and I have a feeling corners were cut here and there. I think I smelled the same smell, faintly, in the laundry room which is on the wall opposite of my bathroom.