My wife's 2005 Expedition 5.4L will rev extremely high for 2 seconds or so every once in a while on startup. Might be a week or more before it does it, but it makes an horrendous sound. I've read about high rpm's at startup but those seem to be more of a normal nuisance type of thing, this seems like a real problem. Any ideas? I should say I think that it's revving high according to her description, it could be something else that makes a similar sound.

 

Ask her how high the tack is when it happens.