While living on a mountain in Wyoming, a neighbor showed me how a flex fan works on a truck - not great. At high altitudes and loads - lower gears / higher revs, just when additional cooling air would be nice - the vehicle slows down and the flex fan flattens out. Not a good match. Just the change to a non-flex steel fan was an instant improvement. Speedway used to have the best price for the steel and aluminum both.
Noise wasn't a problem on my bird until I put the smaller "speed up" pulley on there (to make the waterpump run faster). Many vehicles just don't have the room needed for a fan clutch - but the birds do and that turned down the noise.
Steve Metzger Tucson, Arizona