Replaced Power Supply has not fixed problem!

I own a iMac 27" Late 2009 model (EMC 2374). 1TB HD, I believe it is the i7 model but I am not 100% positive.

A few weeks ago I woke up one morning to my iMac off. I tried to turn it back on but no luck. It took me a few tries with unplugging the power cord and a handful of different methods. I finally got the computer to turn back on. All good.

I backed up everything again just to make sure.

Sure enough a week later it went down again. This time no matter what I tried I could not get the computer to power back on.

One thing I did notice was that whenever i tried to turn the computer on with the power button I would hear a subtle high pitch humming (similar to the bees/wasp sound i have read about.) But that is as far as it would get. The humming would continue until the removed the power cable. Then would start again whenever the power button was pressed. I have swapped power cables and still the same thing.

After doing some research that brought me to IFixIt I was fairly confident I had narrowed it down to the Power Supply. (humming came from top left of screen.)

I have also been unable to see any of the Diagnostic LEDS light up, also making me believe it is the power supply.

I removed the LCD screen and listened closer to power supply and the humming is absolutely coming from that unit.

So I went ahead and ordered a replacement Power Supply unit from IFIXIT. The power supply came and I replaced the unit. However I then went to power the unit back on and nothing. I do not even get that humming sound from this power unit. No Diagnostic LEDS light up. No sound whatsoever. I tried both power cables and a combination of starting methods but still nothing.

I decided to replace the power supply back to the original one. And the humming sound is there on that one so it doesn't appear that anything has changed.

So now I am worried that perhaps the original power supply unit failed and caused more damage to something further on in the computer. I am not the most tech savvy person with computers so I am stumped.

Any ideas on what to check next?

Thank you,

David