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What's with the quiet helicopter sound when using StereoTool with my USB sound card? Bug? It's been there as long as I can remember.
https://www.dropbox.com/s/mdp2w9w0ldlkw ... 9.mp3?dl=0
Changing the low pass setting makes it faster and slower, or makes it disappear entirely. But it shouldn't be there to begin with. I've also heard this sound very quietly on real radio stations during periods of dead air.
Do you have the Quality slider all the way down, and "Ignore high frequencies" enabled? If so, if there is also a DC offset on your sound card, that could cause this. If you really need to run at such extremely low quality settings, you could set the highpass filter to not phase linear, that should get rid of it, at least of what's caused by the DC offset of the sound card.
Now you say that you've heard it on other radio stations during dead air, but that could only happen if they too have a DC offset in the sound card, which seems unlikely (having a DC offset is rare, and it needs to be rather large for this effect to become audible). And they would also need to be running at a very low quality setting.
Do you know if those stations are using Stereo Tool?