i wonder how many flights he has on this, i seem to recall the 1 way bearing in the exi 250 going bad in like 4 to 5 hovers and in the exi 450 it was within 20 i replaced both with align and have yet to have problems but i dont recall the tail twitching before failure. however technically the thwap i hear seems to be the tail catching back up lol and if the 1 way is starting to go it could cause this to happen, to hard to determine.