That sounds like a bug. Did you download and apply the patch? If not, go to the LucasArts homepage and follow the links to the patch. If you have already applied the patch, you may have to start the level over. Sigh!
Hi, Yes that is a bug!! Hornplayer told it right, you have to use the patch and start the last part of the level again. Yeah, you will be scarred when you finally will see and hear the monster, I did! Good luck