Two different idioms, both fairly widely used (not sure about US vs non-US, but I've heard both as a non-American).
'Whistling in the wind' is about futility. A whistle is inaudible in the face of a massive elemental force like wind. "The lone excavator pushed on the hull of the colossal container ship, but it was whistling in the wind."
'Whistling in the dark' is about presenting a brave face in an intimidating situation: a nonchalant whistle in fear-inducing darkness. "They confidently claimed the ship would be moved in two days, I knew they were whistling in the dark."
If you care about understanding people and being understood, simply looking up idioms is way more effective than trying to construct meaning from physical principles. Just a thought :)
'Whistling in the wind' is about futility. A whistle is inaudible in the face of a massive elemental force like wind. "The lone excavator pushed on the hull of the colossal container ship, but it was whistling in the wind."
'Whistling in the dark' is about presenting a brave face in an intimidating situation: a nonchalant whistle in fear-inducing darkness. "They confidently claimed the ship would be moved in two days, I knew they were whistling in the dark."
If you care about understanding people and being understood, simply looking up idioms is way more effective than trying to construct meaning from physical principles. Just a thought :)