How is a window held in?

A year and a half ago, we had all our windows replaced with gas filled double paned. A few months later, there was a big windstorm and one small window made a very slight whistling noise. Everything was so much quieter than it had been with the old windows, and the noise was so slight, that we didn't worry about it.

In our area, high winds happen every handful of months, so we've had a few events since then, and each time the noise has gotten louder, to the extent that now it sounds like someone loudly snoring in the room. It's untenable. The contractor who I used seems to have gone out of business (he had some life events, I didn't think it was a scam).

I noticed that at the bottom of the window there's a gap between the inside surface and the window frame. I pushed from the outside, and the gap closed, but it's easily pushed out again. I assume whatever is supposed to hold it in tight is loose, but I can't figure out what that is. On the outside, there's a plastic molding that's like a frame around the window (to make it brown instead of white), and I'm guessing that has to come off first, but I don't know if I can just pull it out or if there's a trick to it. I don't want to break it.

Any help?