Egnater M4 produces a loud pop when switching channels

[mfred]

Hi guys
Can anybody help on this one? I have an Egnater single channel M4 that produces a loud pop in the speaker when switching channels. This happens when using a footswitch or the button on the front panel. I am hoping that somebody has seen this problem before and could tell me which part to change. Any help would be appreciated.
Thanks
Mike

 

[Bucksears]

Mine does this a little too - it's at Egnater's shop right now getting diagnosed mainly for another issue, but I listed that as well.