[V17.1 Ground combat] Crunching sound on mission start

I've seen mention of it before, but also noticed this on a few maps: sometimes when the ground combat missions starts, crunching sounds can be heard - like there's a spectre underneath the chinook that's being crushed?

For instance in the farm map with the chinook orientation in this screenshot/attach:

I get it on almost every mission with the farm map. I will look to see if I hear it on other missions.