Complex setups with multiple electrical devices often suffer from ground loop problems. This results in 50Hz or 60Hz hum (depending on the local mains voltage frequency) including their harmonics.
Basic tips to avoid ground loops:
- Use balanced cables
Wikipedia article - Connect all of your devices to the same power outlet by using multi-outlet power strips
- Try to find the ground noise causing equipment by disconnecting it
- Avoid to connect unused other devices such as radios or drills to the same power outlet
- Use of a DI box between instrument and audio interface or a ground loop isolator can help in some cases
- Don't let the power cables and signal cable run parallel. If they cross, do it at a degree of 90°
- Never remove the ground pin from any power cord plug.
Additional resources on this topic:
Wikipedia
Sound On Sound