iMac 2011 27" startup beeps

Background:

Replaced CPU (i5-2400 to i7-2600) & SSD upgraded. RAM was confirmed working in the iMac prior to this, and was upgraded to 32gb (4x8gb). I also repasted the GPU while I had the board out anyway. I bought this for cheap as it stopped turning on for the seller (the HDD corrupted the OS and wouldn't internet recovery, got High Sierra back on it with a USB and was confirmed working once that was done).

So basically, this issue started after taking the HDD out, taking the board out and replacing the CPU, and then putting the new SSD in.

Now:

I originally tried to boot it without the SSD as it was still cloning, which gave 3, 3, and 2 1/2 beeps with 3 seconds in between (the last beep of the last sequence was cut off slightly shorter than the prior ones each time, and I could hear the speakers cut out). Now that the SSD is cloned and installed, on startup I now get 3, 3, 2 beeps (definitely no third beep starting to play but then being cut off on the last sequence this time) with 3 seconds in between. Tried to boot into diagnostics - didn't work and showed nothing on screen, just kept beeping. Tried to reset PRAM, didn't work like before.

Running theories currently are that this could be a bad CPU, logic board, possible CMOS/PRAM battery issue or possibly just a loose or incorrectly seated cable, but I'm not sure and don't particularly want to take the board back out if the CPU or board isn't at fault.

Another thing I'm also concerned about that while getting the motherboard out, there was a weird sensor(? I don't know what it is) between the RAM slots and the front bezel. I'm unsure what that is (possibly IR chip from one source I saw, but again I have no sure idea) or if it's important and, if it was damaged somehow, if it could be the issue?

Thanks