Can a plastic ceiling fan box make a fan hum real bad. I have five fans on the same type plastic box an only one is humming real bad. We have installed new fan an it hums too.
Also we took fan from other room an installed it an it hums on this same plastic box.
Would a metal box be better?

You should be using a "Listed Fan Support," not plastic boxes unless they are also listed for fans.

Plastic boxes won't cause hum, but you are in danger of fans falling!

What brands are the fans?