The 360 board has the answer you need. Search "fan". But let me see if I can break it down for you.
Inside the fan room. If you look at your map you'll see a narrow path that is just under the giant fan. There is no floor but there is a grappling hook under there; you just can't see it. What you'll need to do is:
equip the grappling hook. Get close to the entryway, jump down, float your way in, and quickly press R2 when the prompt appears. There you should find another portal doorway. --- Expert at nothing. Amateur at everything. Thinking........Thinking......... IDEA NOT FOUND