Although many windows suffer from being too tight—or worse, completely stuck—another occasional problem is a window sash that is too loose. All wood shrinks to some degree over time, and wood wears down from use. The result is a window that rattles in its tracks.
A sash that is too loose can be tightened. If the gap is not too wide and the stop is nailed rather than screwed, you can adjust the stop slightly without actually removing it.
Score the paint between the stop and the jamb, and place a cardboard shim between the stop and the sash. Protect the stop by holding a block of wood against it, and hammer toward the sash along the length of the stop until the paint film breaks and the stop rests against the shim. Then secure the stop with finishing nails. For wide gaps, reposition the stops.
Another easy fix is to add an insulation strip to the face of the old parting strip. It may take up just enough slack in the window, and it also adds protection against the elements.