The most common cause of this is that the video is a few frames shorter than the audio. This happens to me more often than I would care to admit. The more tracks and the more clips you have, the more likely this becomes because of all the small edits you perform over the life of the project.
Click View>>Timeline>>Zoom Maximum to zoom in as close as possible.
Click Edit>>Playhead>>Jump to End. Check to see if all your tracks end at the same timecode. They probably don't.
If you use groups at the end of the timeline, you may have to open them up and examine the end of the timeline there.