Rifle Lazer Laser

Occasionaly, when im playing fallout 3, I will hear a LOUD lazer pistol/rifle sound that sound like it is being fired at me point blank. There is usually only 1 shot but sometimes as many as 3. When I look around, there is nobody around me, I haave ened popped some Mentads and looked around, but there is nobody. I was wondering, is this a envent I should be aware of or is htis just a random glitch like that jumble of black tumble weed like thing that occasionaly see flying around. Also, does anyone else ever experence either of these 2 listed events?
Neither of these errors affect the gameplay in any way and are just annoying. The flaying black tumble weed generly likes to stay around in one spot and moves through the wasteland on its own slowly so I will occasionlay see it as im wandering around. It seems to hang around map points though and I have almost never seen it out in the middle of nowhere.