We live in a home that is about 47 years old. The laundry room is in the basement. We are on city sewer (not septic). It seems that when I run several loads of wash back to back, we get a sewer odor. This doesn’t happen all the time however. In fact, sometimes we have the odor when I am not doing any laundry. Any ideas on what could be the problem? We do have an open drain on the floor for the Central Air pipe to drain into. Should this be closed off when not in use? Is it possible that we have a clog somewhere?