I had a similar problem yesterday!
In my case the high pitch is hardly noticeable but depending on the my position relative to the Mac, I can notice it
Maybe your noise is more noticeable?
I disassembled it several times and in each step I tried to see where the noise came from, and I can tell you (in my case at least), that it came from the processors! each emitting a different level pitch noise.
I checked if every cable and connector was properly connected and assembled it again, the noise is there, I am not sure if it was like that before disassembly, I will have to ignore it, most will find the noise ridiculous, but I have a really good earing and it bothers me to ear that high pitch.
I tried changing the plug to 180 degrees and no change.
P.S. adding a second drive is really hard, I've disassembled it four times (completely) and it turns easier, although it's still hard to put some parts, but the benefits of a second drive pay off.