Loud popping noises or crackling noises in audio systems often occur when a connection or cable is loose or broken. This issue could be at a microphone connector or somewhere further down the audio signal chain. Focus you attention on connectors which are handled, flexed, or unplugged frequently, and on portions of any audio cables which experience frequent bending (usually near a connector). Any recent changes to your setup are also worth investigating.
Radio frequency interference can also sometimes cause similar crackling sounds. If you are using bodypack transmitters, your wireless manufacturer will be a good resource for helping you track down possible sources of RF interference.
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