is it low frequency or high frequency ?
If it is high frequency it is picking up feedback from a speaker possibly an internal speaker.
If it is low frequency there is two possibilities;
One, the coil is damaged.
Two, it is picking up electronic interference.
To find out which is the case, take your mic and get a cd player or similar device that has microphone input. Take both to another room.
If it is high frequency the noise will stop, if it is low frequency the noise will only continue if the coil is damaged, if the low frequency noise does continue and it turns out to be a damaged coil, it is time to replace the mic. A damaged microphone coil overheats when in use and is a fire risk.
Replacing the coil is an option, but not worth the effort unless you have tons of money to waste, a damaged coil usually though the frequency, amplitude, modulation and resonance properties of other components in the mic out of sync, to re tune the mic to the properties of a new coil is upwards of £200 and unless you go to a specialist most shops will not offer the service as it requires a lot of time and specialised equipment.
You can get a high quality condenser mic for £160
SRC
Studied Music Theory for a year in Birmingham and aside from being a computer, technology and science geek I am a qualified Musician.