The window regulator is an assembly in your door that attaches to the bottom of the window, raising and lowering it in a smooth way. The design of the regulator varies drastically from make to make, and power vs. manual crank. Depending on the individual design, you may see a scissor type mechanism that expands and collapses down, or a cable driven mechanism, but regardless the function is the same. As a regulator ages, the moving parts won’t move as smoothly as they once did. You may notice the window knocking, scraping, or grinding as it moves, signalling part of the window regulator is about to fail. Commonly when a window regulator fails, the window falls down into the door frame. To fix, you must take the door panel card off (usually held in with a series of removable body clips and a few bolts) and work on it through the access holes. Occasionally, you can get away with just replacing the highest wear part of the window regulator, the window regulator rollers.