Crackling in speakers

SJMcD

Hi Everyone,

I was wondering if someone could help me with an issue I have.
I am a newbie when it comes to mixers, so for some of you this may be a silly question and the answer is obvious.
I have a Soundcraft u12, which has one speaker plugged into the main output, 1 speaker plugged into Auxiliary and 1 microphone as an input. Recently crackling has been coming out of both speakers even when I have the main volume turned to 0. I have replaced the lead from the microphone to the mixer, replaced the microphone, unplugged each speaker one at a time, but still have crackling coming from the speaker plugged in. Can anyone give me any ideas what the issue might be ?