I'm assuming you've already checked your monitors and they are OK.
Check your cables.
Get some "DeoxIT D5"(improves conductivity,deoxidizes, seals and protects connectors and contacts) or something similar and spray it on all your jacks and connectors. Follow instructions on label.
You can also use deoxit to clean pots and faders. Another source of noise.
If this fails to eliminate the noise it could be a failing mechanical connection,
radio interference from another power source or a failing amplifier.