Try disconnecting the trackpad flex cable from the main logic board (shown in Step 23 of this guide). With the cable and battery disconnected and the bottom plate removed, open the display and set the machine on its side with the MagSafe 2 port facing upward. Then connect the MagSafe charger and press the power button. If it’s able to power on the issue may be due to a failing trackpad flex cable or a failing trackpad. If that doesn’t help, you’ll need to do some further troubleshooting with the I/O board and the logic board as the point of failure probably involves one of those two components. Hope this helps!
Not sure if this will help but here is a similar question. It seems to be for a different logic board spec but the same general model of iMac. Check out the answers there and you may find the information you’re looking for!
If you inspect the two logic boards (Board 1 and Board 2) for differences in these pictures, you’ll notice that many of the cable connections to the logic board are of a different style or in different locations. This means you cannot swap the boards in these machines.
Can you verify that you installed a 2018-2019 display? If you check here they mention the following: “2017 Screen will work on 2018 model but it will glitch during boot + no truetone.” That sounds similar to your symptoms. Otherwise I would be inclined to think the display you got has a defect.
From the video it sounds like the noise could be the result of something underneath the power button keycap, but that would not explain your other symptoms. Where does the noise seem to come from within the machine? Is it loudest near the power button itself or near some other component? The only other moving part in your system is the fan, so it might be a good idea to remove the bottom panel, open the machine on its side, and observe the fan when hitting the power button. Another possible theory is that when you're trying to power on the machine, the system is actually already on but in a frozen sleep state. So in other words, when you hold down the power button to start it you're actually forcing the frozen system to power off (Mac has a feature where if you hold the power button for about 5 second it forces the machine to power off). Forcing the machine to power off like this sometimes produces a click noise (I think this is because the fan suddenly stops). You can check to see if the system is...
Try disconnecting the display cable, inspecting the pins both on the cable side and the port side for damage, dusting them with compressed air, then carefully reseat the cable. If that doesn’t help, you can try replacing the cable itself (cheaper than replacing the display). In my experience though, this specific visual glitch is normally due to damage to the display assembly itself and not an issue with the display cable. It may also be worth checking with an external monitor to verify the issue is not with the graphics, but rather the hardware of the display. My best guess is that the display assembly itself is damaged and would need to be replaced.
Someone posted a very similar question a while back. They decided to replace the display assembly and this fixed the issue. Here’s the other internal display only works when an external display is connected.
In Step 4 of this teardown you can see that the LCD cable in the 2019 differs from the 2017. You can find the 2019 displays from individual sellers online, here’s a link to an eBay post. Hope this helps!