I have a lovely Fishman SA220 Performer, used it for years and love it to bits.
https://www.amazon.co.uk/Fishman-SA220-Solo-Performance-System/dp/B001E96UMS
At rehearsal last night it started making a fairly constant hissing/crackling sound, loud enough to be bloody irritating when not playing but inaudible when in use.
The sound was completely unaffected by whether or not an instrument or mic or both was plugged in, and none of the controls (gain, EQ, etc.) made any difference to the sound.
I realise that this is damned near an impossible question to answer without hearing it, but does this sound like a capacitor or similar?
My problem is that, in perfect condition, this unit is currently worth perhaps £400, maybe a bit less. If I send it off to be serviced and repaired, it's hard to believe that the bill will be a great deal less than that ...