This article describes a buzzing noise the iMac (Slot Loading) may make after shut down.

A very small number of iMac (Slot Loading) computers may produce a buzzing or humming noise from the speakers after shut down. The noise can last for several minutes and will go away if the power cord is unplugged from the computer.

If your iMac (Slot Loading) makes a loud buzzing or humming noise after shut down, contact Apple or an Apple-authorized service provider for assistance in resolving this issue.