- Check the source you are plugging your microphone into.
- Switch your microphone to a different input, if one is available, to see if the problem persists on a different channel
- If so, try switching the power source of your interface/preamp, possibly to one that doesn't have many other things plugged into it
- Try a different preamp or interface to see if the problem persists on a separate unit
- If using a USB audio interface, try switching the USB input on your computer
- Switch your microphone to a different input, if one is available, to see if the problem persists on a different channel
- Try using a different mic cable; there is a chance your cable is bad
- Make sure your XLR cable isn't running parallel to any power cables, which could cause a hum from signal interference
- Plug in a different mic and see if you get the same noise
- Use the same cable to determine if the noise if coming from the microphone itself