How much Sound Sources do you have set in your Audio Settings? Try setting it high or lower to see if it fixes the issue.

Doesn't fix the issue. It still sounds weird ("wooooaaawoooaaowoowoaaaawwoaaaaa") like in the video.

Oh I see. Its a bug then.

Not a bug and not enough variation in the samples.

This is not a bug, great video explanation though. I see exactly what you are talking about. The echo effect would be a simple fix. I will vote this up, even though I would never expect to see so many vehicles so close. Maybe the Devs could just use the "Louder" ambient rain for the inside of the vehicles only, or just at point blank range.

Just watched and listened to the video...

this is due to each helicopter emitting its own rain impact sound (which is good while stood still)

But ARMA 3's exaggerated 'doppler effect' as you're approaching one sample becomes higher in pitch (compressing the wavelength) than the sample that you're walking away from (stretching the wavelength) causing the two (or multiple in this case) wavelengths to go slightly out of phase.
resulting in the strange whooshing sound that you're hearing...

Maybe to fix this they could disable the Doppler effect for walking and running speeds as it isn't really noticeable in real life.

Well you can hear something is off even with like 4-6 vehicles in a base but it's naturally not so easily heard.

I was able to notice it. Its like an eco like sounds.

Just noticed the issue as well, even after all the audio changes etc
https://www.youtube.com/watch?v=-CjhojiCsf8