iMac Late 2009 - Black screen strange noise after startup

Hi gang,

I've been on the road towards rehabbing my Late 2009 iMac. I've replace the power supply (because the old one was frying logic boards) and replaced the mother board with one I purchase off ebay, and I've replace the original hard drive with a new 3 TB drive from OWC that also includes a thermal sensor that's place upon the hard drive, which has a cable that plugs into the hard drive thermal sensor port on the logic board.

At this point I've installed the current replacement logic board. Without the LCD screen plugged in, it boots with no issues it. With the LCD screen plugged it, it boots and then makes a strange digital sound. The screen remains black. The fans and hard drive run. I've checked the Diagnostic LEDs at the bottom. Only lights 1 and 2 are lit. According to my official Apple tech guide, this means that it has power and the system is using it. Had lights 3 and 4 been lit, the system would had been communicating with the video card and it would be connecting to the LCD screen.

My next basic step is to connect an external display when I finally receive the adapter in the mail this week. (Hopefully it'll be the right one.) This will let me know if it's the video card or the LCD display. If the issue is with the video card, it seems like I'll have to replace the logic board again. I've take apart one of my dummy boards (a fried logic board, my original logic board), and after detaching the heat sync, it looks as if it's been glued on there by Apple. Is there any way to safely detach the video card from the logic board?

I still haven't found anything via Google or anywhere else on the noise that it makes after it boots. I was hoping someone here on the Answers Forum might have an idea as to what's causing it. My best guess is that it has something to do with the video card.

Thank you in advance,

Chris

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Here's an update as of 10/31/16:

Tried connecting an external monitor via the mini-port, and nothing. So I'm assuming it's an issue with the graphics card.

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As of 11/03/2016

I've connected an external display with out the iMac's LCD screen attached. I hear the normal boot sound, but I can still get no picture on the external monitor. With a multimeter, I've tested the test point, and there was no voltage in point 13. That means that there is no voltage going from the logic board to Backlight Controller Board. Does that mean the Backlight controller board needs to be replaced or is the issue with the graphics card?

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