Jimmy_Blue

I couldn't find anything when I did a search but I just bought GR:AW in a bundle with Rainbow Six Vegas and started playing yesterday - and I've noticed that every now and then a wierd noise occurs and can't find out why - the noise sounds like the sort of sound you'd hear when pulling something out of a plastic tube. I haven't yet been able to associate the sound with something in game so I am not sure if it is a bug or an indication of something happening. It sounds like a cross between a boing and a pop. What am I missing? What it could almost be I guess is the sound a mortar round makes coming out the tube - only I hear it when nothing like that is happening. Any ideas?