Yes, my Ultra is very quiet. Absolutely no high pitch sound no matter how close I put an ear.
I don't know what your distinction would be between production inconsistencies and build inconsistencies? That would be the same thing in this engineer's book.
I haven't seen enough evidence to positively prove whether the "whistle" is from a power supply component or faulty fans, but I'm leaning toward a batch of faulty fans. It seems that the M1 Max is much more susceptible to the problem than the Ultra, but since both the fan/cooling system design and power supply current draws are different, that doesn't prove which it is.
However, since the fans don't speed up under power supply load, and no one has indicated the "whistle" changes pitch or volume with changing power supply loads, it doesn't appear to be a power supply component. And there have been reports that changing the fan speed manually (with fan control apps) does affect the "whistle" volume or pitch, and that points to a fan problem.
Early on, there were reports of the "whistle" in units manufactured in China, and not Malaysia. But most people don't report where their problem units were manufactured. So I don't know if that is still true or not. But if it is, that could also point to a fan batch problem, but not necessarily since we don't know if both countries got fans from the same batch or not.