A whisle is most likely a vacume leak. The best trick I know of to find it is to spray carb cleaner around the base of the carb and the intake manifold gaskets.[the most common places for vacume leaks]
If the idle goes up when spraying the carb cleaner in an area there is your leak.
My BBK 70 mm whistles at idle, too. It is apparently a common problem. The cause is purported to be the idle air bypass ports in the throttle body. They have an unfinished, sharp edge where the port intersects with the throttle bore. The fix, which I haven't done yet, is to remove the throttle body and smooth, or round-off, those edges in the throat of the throttle body with a fine file or fine sandpaper.
Is at least one valve cover vented; ie open to atmosphere with a breather (without a PCV valve) or hooked to the elbow on the throttle body (again without a PCV valve)? My breather valve had a PCV valve in it and engine whistled as you describe, at idle, and for a few seconds when I shut the engine down. Replaced the breather with one that had no valve and whistling stopped; two much vacuum build, I think.
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