I have 2 bathrooms in my house -- one on the main floor and one on the second floor. I haven't noticed any odors on the second floor, but I can smell something coming from my main floor sink. It only happens when I turn the water on and goes away when the water is shut off. I've also noticed that when using the toilet, the smell seems to appear. Whether it's coming from the toilet or from the sink at that point is hard to tell.
I've used Septo-bac a few times but this doesn't do much, if anything. I've heard that this can happen if the water in the trap dries up, but the sink is used quite often and I can see water when I look down the drain.
I've heard bacteria can build up in the overflow drain, but this is an old sink and doesn't have an overflow.
Other thing I've noticed is this drain is somewhat slow moving and tends to clog on a regular basis.
What should I look at next?