"M' is supposed to call up the Track Manager. Do any of the tracks have an unchecked checkbox next to them? If so, they would be "hidden" tracks. Check them off and see if maybe they extend past the end of your song.
A node is a point on an envelope. Are you using any envelopes? If not, you don't need to worry about nodes.
As Aaaron stated, you probably have *some* type of data extending past the end of your song. The trick is to fgure out what it is.
The other thing you could try is to go to the end of the song. Left click and hold on the time ruler and drag it to the right until you reach the end of the track. Release the mouse button and make sure ALL the tracks are highlighted over the area you just traced (but also make sure NONE of your actual music is highlighted). The go to Edit > Delete (make sure the delete hole checkbox is checked) and OK.
See if that does it for you.