99 Explorer Sport, loud noise, fluttering idle

My 99 Ford Explorer has started emitting a loud noise (like a vaccuum cleaner only louder), and if I put my truck in park, or foot on the brake and stop, (while noise is happening) the tack flutters between 0 and 1 then my truck dies. I can re-start my truck and it's fine, this problem is intermittent, happens wether my engine is cold or warm. The noise problem has been happening for awhile, my engine cutting off is a new variable of the problem. My mechanic can't seem to find a problem, and since the problem is intermittent I can't bring it to him exactly when it happens. Any suggestions???? Thanks.