Basement

Get rid of the mice! :-) Seriously, basement odors can be stubborn when there isn't a cross ventilation to get rid of them. You could use bleach along the sides/walls to kill the germs that cause odor. Then hang bags of cedar chips from the rafter in each part of the basement. Mice will come in for food, warmth, and water. Keep up the preventive measures for mice - no food open or in paper containers, etc. Do your best to keep the basement dry. Use mouse poison, traps, etc. to keep them out of there.