We have a musty odor in our home?

We have a musty odor in our home. We had mold remediated by a reputable company and the air samples came out clean after the remediation. We moved from the property, have replaced all the carpet, and painted all the walls. We have had our furnace serviced and the crawl space lined with plastic and new ducts put in. The crawl fludded several months ago, but everyone who has been down there says the duct work is okay. There is a fireplace in the home. We are trying to sell this home. Does anyone have suggestions as to where the smell could be coming from?