What happens when I turn my PSU upside-down?

Turning it upside down won't stop any noise. Take a look inside the vent with a flashlight and see if there are any wires or any debris that the fan is hitting. Sometimes an internal wire is routed wrong and will brush against the fan. And sometimes there is debris that can get inside. On some models you can take the back plate off to remove any debris or any foreign objects that might have gotten in there.