Real quick test to see if it’s sucking air: pull the oil fill cap out of the valve cover and run it. If it goes away, it’s sucking air.
I think that sounds like a metal on metal squeal. Like blastservices said, I bet it’s the inner end of the balancer on the metal behind the cover seal. Likely...
I thought the notch was just to get over the key but no locating beyond that. Can you measure where that black mark was to the inner end of the balancer? Basically try to take some measurements to ensure the balancer is seating far enough?
Sounds like you’re going at it how I would. When you have it apart, check to make sure there’s no marks anywhere on the cover or anywhere else, as mentioned earlier, where something could be rubbing.
Long shot thought: do you have pcv hooked up and the inlet vent in valve cover isn’t plugged...
I lube lightly with wheel bearing grease. Are you loosely installing the timing cover, installing the balancer and then tightening timing cover bolts? If not, the seal can be misaligned with the balancer. If you are doing that, disregard.