Its been years since I've played FFVII but I recall that I always do the Fort Condor before Corel (cause I'm lazy, Fort Condor is nearer Mideel). From What I can remember, after doing the Huge Materia in FC and Corel, I went to Mideel, talked to Tifa then Cloud, then he starts moving his head up and down, then there's an easrthquake, and some weird shrieking sound.