The dominant sources of noise are:

• seismic noise 
• mechanical vibrations carried through the earth
• transmited into interferometer via infrastrructure
• caused by plate tectonics, ocean tides, cars, ....
• thermal noise
• atomic and molecular motion in interferometer associated with non-zero temperature
• shot noise
• statistical fluctuations in the number of photons measuring mirror position
• see Ed Daw's talk next week!