That could be a collapsed pvc hose on the back of your engine. Look at it if that's the problem here is a how to.
http://www.focusfanatics.com/forum/showthread.php?t=190218
I've been on here a lot and I've fixed a lot of theirs problems myself so I know what thread to look for lol I hope this helps.
But if that's not it I'll bet it's the alternator.