it depends on your equipment and your house...
1. electricity oscillates at 50-60Hz so your setup may be seeing some dirty power that is translated into sound. usually this isn't the case, but you can test by testing your equipment on a power source far enough away from your home that it's fed from a different power grid.
2. if you're using your computer, it may be translating activity into sound. there's a lot going on inside a computer, between the power supply, cooling fans, and hard disk movement.
3. if you're getting significant static, however, (and i assume you're smart enough to rule out airflow on the mic) then it's likely your gain levels, your mic itself, or your cords.
-- gain levels would be the intuitive choice for your problem. if you have a jacked up gain anywhere in the chain, that would account for clipped sensitivity and even a pin dropping may cause a static blip. rule of thumb is half the max volume level, or back off until you hear no feedback. this would usually be the cheapest first step in troubleshooting (after checking cords and disconnecting/reconnecting it all).
-- last, it could be your mic. microphones can get blown just like speakers (albeit differently) and it's possible that your mic isn't healthy. easy test would be to plug in another input device and determine if the static remains.
-- as limn pointed out, crossing cables (and sometimes even proximity), especially power cables and signal cables will often cause static. likewise, signal cables could have oxidation inside (or someone could have played jumprope with them).