As far as I remember this is the correct tunnel you're talking about. I did not hear about that problem yet.
Workaround: you could temporarily become invulnerable (hit F10 and type taklit_marion on), now jump down and restore vulnerability (F10 taklit_marion off).
Nevertheless, make sure you have the Update 1.2 installed. If not, go get it at ftp://ftp.lucasarts.com/patches/pc/J3DUpdate1.2.exe - you'll get an EXE file which you simply double-click, it installs itself.
Unfortunately, after installing the Update 1.2 it is necessary to replay the entire level 16 beginning with your last savegame of level 15. This is at least what most people here in the forum have experienced.