The breather box is at the bottom of the hose/tube which comes out of the bottom of the flame trap assy. It's buried on the block just above the oil pan.

The whistle only occurred at one set of conditions, opening the throttle, which induced high flow into the manifold. I could also tell that it was coming from the area at the back of the engine.
Checking the plugs was the real clue, #4, at the back was normal, all the others were way sooty.
If you have a constant whine or whistle, I'd use a long tube or fairly large ID hose to listen around the engine compartment to figure which part is making the noise.
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