Actually much of that sound is caused by several Poltergiests tossing things around. There are at least 5 of them in X18. Shooting at the hovering balls that look like electro anomalies will kill them and reveal their true form. When the Poltergiests and Psuedo Giant are killed the place becomes as quiet as a church.
For an interesting experience try taking the Kill the Mercs quest offered by one of the guys in Bar (inside the bar). If you time it so you go there just when it's getting dark it is very atmospheric. There is lightning illuminating the dark sky and a lurking Poltergiest that likes to throw explosvie barrels when he gets near them.
The first time I did that quest I had just started using some good mods. I went in all confident I would be able to snipe the Mercs with my silenced rifle then this Poltergiest decides to make it hard for me to accomplish that. I recommend doing this with a good realism mod that includes Darker Nights.
Hey Frag Manaiac :) LOL you know, I wondered what those things were!! I pretty much ran through that entire area... as the first time I played it and met that monster at the tunnel (the one with the jeans that makes your screen go all crazy) really freaked me out. I've since completed S.T.A.L.K.E.R. (for the first time) and, well, it wasn't a happy ending, so I presume I've done something wrong. I'm currently re-playing it through with the weather overhaul mod 2.0 (which I highly recommend):
http://board.oblivion-lost.de/showthread.php?t=12303
And wow, it's really beautiful... in an ugly, post-apocalyptic, sort of way... I'll take your advice and go for the darker nights option. Though even the brighter option is pretty freaky... I'll definitely be trying out some of the other quests! Cheers for the heads up, I could live in this world forever :)