We've had similar trouble with the fans on a YCAV0157 and YCIV0157. The fan blades are coming in contact with the fan shroud. I attended a JCI YCAV class recently, mentioned this to the instructor. His response is that the early shrouds were fashioned by hand as opposed to being machine stamped and were not coming out precisely round. His advice was to take a hammer to the shroud and pound out the areas where the blades are hitting. Haven't quite gotten around to doing that yet.
Just tell 'em I meant well.