Have you ever stuck your head inside a Tower PC to hear the noises in them? Many drives makes these noises yes it a bit annoying if you sit and stress and obsess about it but I don't hear mine anymore, its like having a extractor fan fitted in your home, its annoying as **** for a few days then you tune out, if you sit listening for it you will hear it, but why worry, its how they sound, the drives work, even my late 2009 WD made noises, its on a desk in a small enclosure in front of you, you will hear it if you listen for it.
You have your choices but to go through 4 iMacs should say that you are either looking for perfection you will never get, or you need to go back to a Windows box on the floor as this noise is very common in iMacs, believe me if you heard some of the noises other AIO machines make, well the fan noise on one brand would drive you insane, try getting Apple care and living with it, soon you wont notice it, or move on and get on with your life.
Sorry to be blunt but Apple machines are like any other computers, unless you have an SSD you will hear mechanical sounds, they are not perfect they are mass produced, nothing is perfect in life, but if you used your machine instead of stopping to listen for the noise all the time you would maybe enjoy your Mac?