Squeal when starting
 

A buddy of mine has a 5.0 5spd Stang and when he starts the car it makes a squeal, but it only happens when the car gets started. He replaced the serpenpine belt and it's still there. What could it be? And could it be a big problem?

sounds like the tensioner is going bad or he has something that is on its way of seizing up. e.g. water pump, A/C compressor, smog pump....

The car also turns over slowly a couple times before it catches and starts up. But usually when it's doing one thing, it won't do the other.

The sqealing is probably a bad tensioner or the bearing itself, also this is also a symptom of an air pump starting to lock up, borrow a short belt or if you have your own eliminate it and see.

Also, not to insult you, but did you get the exact length belt and the proper ribs.

Battery good?

This isn't me, it's a buddy of mine. As far as I know the belt is the correct length. Sorry I don't understand what you meant by a bad bearing (tensioner bearing?). Eliminate what?? the Air pump?? How would he do that?

I agree with the bad bearing diagnosis. Remove the belt and turn the accessories by hand and you should be able to feel the roughness in a bad bearing. All the accessories should turn fairly freely, with the exception of the power steering pump which will be stiffer. What gtsr515 means is get a shorter belt and bypass the smog pump to see if it still squeals on start up.
For what its worth I have also seen ( heard ) alternators do this on start up, so check out all the accessories.