Can someone please tell me why the water in our downstairs bathroom sink has a funky sewer smell when we turn on the water? Of note, we live in a split level home, so the downstairs is actually considered a converted basement.

The downstairs sink seems to be the only watersource that has this problem in the house. The shower in that same bathroom doesn't have that problem nor does the water that is pumped through our washer which is also downstairs. It also seems that the smell is more noticable when we turn on cold water. After letting it run for a while the smell seems to lessen. In addition, none of the water sources upstairs has this problem either. It is really bothersome as we have to go all the way upstairs to the kitchen if ever we get thirsty and want a drink of water! Any suggestions?