Don't just replace the filter. Clean out the area where the filter is placed. Moisten a rag or paper towels with 409 or a similar general-purpose cleaner, stick your arm down the hole, and clean the area as thoroughly as you can. You may even be able to reach the fins of the fan. You'd be amazed how much disgusting sludge comes off on the rag, especially if you neglected the filters and did not change them regularly. Remember, this is a mildew farm. Also spray Lysol or a genuine AC cleaner down there.