What can cause fumes to come from basement even if the heat is not running. It slowly leaks upstairs and seems to be worse when we are gone and come back home. The basement has the furnace, but the heat is not running, but the air is running. We know we have a mold problem and that is evident from the musty smell. But the other smell is like fumes. It doesn't appear to be coming from the furnace...It is a rental home. But we cannot tell what that exhaust fume odor is. It is worse in basement. It has been this way since we moved in 5 months ago. I am home most of the day and I don't want to get sick from this.We have no garage and have oil heat and central air. It is an older home built in 70 and has some electrical problems I think. The lights blink a lot. It is a rental home and we are moving in 3 weeks, but I am kind of scared to even stay the 3 weeks.. I am home most of the day and my husband is at work most of the time. I have been kind of sick and not sure if it is related to the house. It could be the allergies in a new location.