First question, what is the black pipe in the picture above where the drain pipr goes into the wall?
Additionally, we have an intermittent sour smell coming from this bathroom. I replaced the wax seal of the toilet thinking that might be the problem. However it was not. Last night, the smell occurred again. After some investigation and wafting, i think I narrowed the smell to underneath the sink (hence the picture and post).
I guess the second question would be how do i get rid of the sour smell?