In reply to FesteringSore:
Its territorial/ intimidating fighting talk.
It works too, I saw gorillas in Uganda. The silver back on first acquaintance beat his chest quite violently and charged us. I didn't argue with him, but was intimidated and respected the fact this was his territory. He didnt need the charge to make his point.
After we made friends, he laid on his back, rolled the chap back and ate the cheese. All quite delicately and slowly.
Wild gorillas it seems, like their captive counterparts, have no dignity either.
I also have seen an "ook". He wasn't in a library, or riding on a giant turtle/ elephant combo as far as I'm aware, but did have bright green turds that whistle past you on their descent. I suggest caution when in a jungle and looking up to strange noises.