Dirt and Dust around blower unit in basement
The more activity you have in a home, the more dust you will have. I once serviced a maintenance contract at a gym, and their air filters got filthy dirty in two weeks time.
The dust must be migrating towards your blower because of the suction it creates. Usually, the return is ducted down to the blower from upstairs. Is this return open and unobstructed? Air filter clean?
It is okay to return air from the basement, but I wonder if your equipment is getting dusty and dirty inside also. If air is getting into the system somehow without going through a filter, you could have problems with the blower and coil getting clogged.